1. An input system of an electronic device based on Chinese phonetic notations, the input system comprising: a plurality of keys respectively arranged on a first side and a second side of a screen of the electronic device, wherein each of the keys corresponds to two phonetic notation sets, and each of the phonetic notation sets comprises a plurality of Chinese phonetic notations including one to two Chinese consonant notations and either one to two Chinese vowel notations or a Chinese medial notation; a detecting unit coupled to the keys, wherein every time the detecting unit detects an operation on one of the keys, one of the phonetic notation sets corresponding to the operated key is chosen according to a direction of the operation; a filtering unit coupled to the detecting unit for permuting all of the Chinese phonetic notations in the chosen phonetic notation sets to obtain at least one candidate meaningful pronunciation; and a candidate character output unit coupled to the filtering unit for outputting at least one candidate character onto the screen according to a Chinese character database and the at least one candidate meaningful pronunciation.