1. A Chinese pinyin input method by using only numerical keypad, and implemented by a device, which has a numerical keypad, a memory portion and a display portion, for inputting Chinese characters, comprising the following steps: Step 1: arranging pinyin symbols according to the arrangement of the English letters on the numerical keypad of a general telephone set, assigning one of the keys, excluding the keys “2” to “9”, as a conversion key, and constructing a key and pinyin symbol combination look-up portion in the memory portion for storing keypad key signal combinations and corresponding pinyin symbol combinations in accordance with the keypad arrangement; Step 2: constructing a dictionary in the memory portion for storing the Pinyin symbol combinations and corresponding Chinese characters of words; Step 3: based on the aforesaid look-up rules for keys and pinyin symbols, inputting a key signal by pressing a key probably followed by additional conversion selection keys from the numerical keypad according to the desired pinyin symbol; Step 4: according to the key signal inputted by the user, combining the inputted key signal with previous inputted keystroke signals yet to be converted into a Chinese character, and retrieving the corresponding pinyin symbol combination from the key and pinyin symbol combination look-up portion; Step 5: analyzing the corresponding pinyin symbol combination, repeating Step 3 if there exists a plurality of combinations for continued input by the user, otherwise, comparing the pinyin symbol combination together with a string of pinyin symbols inputted beforehand with the dictionary to retrieve the corresponding Chinese character of word, and transmitting the corresponding Chinese character or word to the display portion for output display; and Step 6: unless terminated by the user, repeating Step 3 to continue with input of a next Chinese character.