5. A speech assessment method for using the speech assessment device for the multisyllabic-word learning machine as claimed in claim 1 , comprising the following steps: a step of starting the assessment mode including using the central processing system to let the displaying unit displays the playing interface; a step of selecting words to be assessed including using the playing interface to select the standard continuous audio files for multisyllabic words, or the standard audio files for each separate character of the multisyllabic words from the standard speech database; a step of choosing to play or record including choosing a step of playing or a step of recording; a step of recording including using the speech receiving unit to receive and convert the words to be assessed spoken by the learner into the learner's continuous audio files for multisyllabic words, and the learner's audio files for each separate character of the multisyllabic words, wherein the learner's continuous audio files, and the learner's audio files are stored in the learner's speech database, and also transmitted to the audio visualization unit to create the learner's monosyllabic reference polygonal line for each separate character and the learner's continuous multisyllabic reference polygonal line for multisyllabic words, by conducting a step of visualization; the step of visualization including: a step of picking out fundamental frequency, a step of defining analysis point, a step of transforming polygonal lines, and a step of simplifying the polygonal lines, wherein the step of visualization is capable of simultaneously converting standard audio file into the reference polygonal line and converting the learner's audio file into the learner's polygonal line; the step of picking out fundamental frequency including selecting the most stable section of the frequency of the words spoken by the learner, picking out the fundamental frequency of the stable section, and forming a learner's initial curve corresponding to the first character of the multisyllabic words spoken by the learner, wherein the horizontal and vertical axes of the graph of the learner's initial curve are defined as a time axis and a frequency axis, respectively, the step of visualization is performed character by character to form the learner's monosyllabic reference polygonal line of the learner's monosyllabic reference polygonal line for each separate character; the step of defining analysis points including defining that the learner's initial curve includes a plurality of analysis points; the step of transforming polygonal lines including forming at least one section of line by connecting consequently connecting the four analysis points to one another, defining the section of line as an initial comparison syllable, so that the learner's initial curve presents a line which consists of the at least one section of straight line formed by the initial comparison syllables; the step of simplifying the polygonal lines including comparing or checking the durations of the initial comparison syllables, and the total duration of the learner's initial curve, if the duration of the first initial comparison syllable is found to be less than 30% of the total duration of the learner's initial curve, the first initial comparison syllable is considered as unrepresentative and then combined with a neighboring second initial comparison syllable to form a combined initial comparison syllable, then the combined initial comparison syllable is connected to another representative initial comparison to form a learner's monosyllabic reference polygonal line for bi-syllabic words; a step of repeating including: repeating the step of picking out fundamental frequency, the step of defining analysis point, the step of transforming polygonal lines, and the step of simplifying the polygonal lines, so as to convert the rest characters into the learner's monosyllabic reference polygonal line for bi-syllabic words of the learner's monosyllabic reference polygonal line, and connect the learner's monosyllabic reference polygonal line to form the learner's continuous bi-syllabic reference polygonal line which is to be displayed on the displaying unit; a step of assessment including picking up the monosyllabic reference polygonal line for each separate character, and the continuous multisyllabic reference polygonal line for multisyllabic words, and comparing them with the learner's monosyllabic reference and the learner's continuous bi-syllabic reference to form a plurality of comparison parameters, and the step of assessment include one or more comparison parameters.
5. A speech assessment method for using the speech assessment device for the multisyllabic-word learning machine as claimed in claim 1 , comprising the following steps: a step of starting the assessment mode including using the central processing system to let the displaying unit displays the playing interface; a step of selecting words to be assessed including using the playing interface to select the standard continuous audio files for multisyllabic words, or the standard audio files for each separate character of the multisyllabic words from the standard speech database; a step of choosing to play or record including choosing a step of playing or a step of recording; a step of recording including using the speech receiving unit to receive and convert the words to be assessed spoken by the learner into the learner's continuous audio files for multisyllabic words, and the learner's audio files for each separate character of the multisyllabic words, wherein the learner's continuous audio files, and the learner's audio files are stored in the learner's speech database, and also transmitted to the audio visualization unit to create the learner's monosyllabic reference polygonal line for each separate character and the learner's continuous multisyllabic reference polygonal line for multisyllabic words, by conducting a step of visualization; the step of visualization including: a step of picking out fundamental frequency, a step of defining analysis point, a step of transforming polygonal lines, and a step of simplifying the polygonal lines, wherein the step of visualization is capable of simultaneously converting standard audio file into the reference polygonal line and converting the learner's audio file into the learner's polygonal line; the step of picking out fundamental frequency including selecting the most stable section of the frequency of the words spoken by the learner, picking out the fundamental frequency of the stable section, and forming a learner's initial curve corresponding to the first character of the multisyllabic words spoken by the learner, wherein the horizontal and vertical axes of the graph of the learner's initial curve are defined as a time axis and a frequency axis, respectively, the step of visualization is performed character by character to form the learner's monosyllabic reference polygonal line of the learner's monosyllabic reference polygonal line for each separate character; the step of defining analysis points including defining that the learner's initial curve includes a plurality of analysis points; the step of transforming polygonal lines including forming at least one section of line by connecting consequently connecting the four analysis points to one another, defining the section of line as an initial comparison syllable, so that the learner's initial curve presents a line which consists of the at least one section of straight line formed by the initial comparison syllables; the step of simplifying the polygonal lines including comparing or checking the durations of the initial comparison syllables, and the total duration of the learner's initial curve, if the duration of the first initial comparison syllable is found to be less than 30% of the total duration of the learner's initial curve, the first initial comparison syllable is considered as unrepresentative and then combined with a neighboring second initial comparison syllable to form a combined initial comparison syllable, then the combined initial comparison syllable is connected to another representative initial comparison to form a learner's monosyllabic reference polygonal line for bi-syllabic words; a step of repeating including: repeating the step of picking out fundamental frequency, the step of defining analysis point, the step of transforming polygonal lines, and the step of simplifying the polygonal lines, so as to convert the rest characters into the learner's monosyllabic reference polygonal line for bi-syllabic words of the learner's monosyllabic reference polygonal line, and connect the learner's monosyllabic reference polygonal line to form the learner's continuous bi-syllabic reference polygonal line which is to be displayed on the displaying unit; a step of assessment including picking up the monosyllabic reference polygonal line for each separate character, and the continuous multisyllabic reference polygonal line for multisyllabic words, and comparing them with the learner's monosyllabic reference and the learner's continuous bi-syllabic reference to form a plurality of comparison parameters, and the step of assessment include one or more comparison parameters. 9. The speech assessment method as claimed in claim 5 , wherein the unrepresentative initial comparison syllables in the step of simplifying the polygonal lines are combined into a combined initial comparison syllable by performing the following steps: selecting a modified fifth analysis point at the center of the unrepresentative initial comparison, connecting the to-be-combined analysis point to the fifth analysis point, and extending to a position which is at the same point as the first analysis point along the time axis to form a sixth analysis point.