17. The speech assessment method as claimed in claim 14 , 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.
14. A speech assessment method for using the speech assessment device for the multisyllabic-word learning machine as claimed in claim 1 to visualize a continuous audio, comprising the following steps: a 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; a step of defining analysis points including defining that the learner's initial curve includes a plurality of analysis points; a 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, 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; a 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 is found to be less than 30% of the total duration of the learner's initial curve, the first initial comparison is considered as unrepresentative and then combined with a neighboring second initial comparison 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; and 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 lines, 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.