1. A speech recognition system with cepstral noise subtraction, comprising: a filterbank energy extractor, for obtaining a plurality of first feature vectors according to a voice signal; a cepstral noise subtraction device, for obtaining a first feature vector of a preset voice frame and first feature vectors of a plurality of voice frames before the preset voice frame, so as to calculate a feature mean vector, and calculate a second feature vector of a preset voice frame according to the first feature vector, the feature mean vector, a first scalar coefficient, and a second scalar coefficient of the preset voice frame; a cepstral converter, for converting the second feature vector of the preset voice frame into a cepstral feature vector; a model trainer, for calculating a model parameter according to the cepstral feature vector; and a speech recognizer, for calculating a recognized voice signal according to the cepstral feature vector and the model parameter.