Aiming at the problem of musical noise in dereverberation of spectral subtraction, a post processing masking method based on frequency normalization is researched. Firstly, the reverberation signal is processed by spectral subtraction to remove the reverberation. Secondly, the reverberation signal and the dereverberation signal are normalized in frequency. Then the dereverberation signal and the reverberation signal are subtracted correspondingly in frequency domain to get the mask factor. Finally, the mask factor is used to deal with the dereverberation signal in order to reduce the musical noise. In the simulation experiment, the spectrogram and the perceptual evaluation of speech quality are used to evaluate the masking method. The spectrogram verifying the musical noise is reduced obviously in the post processed speech signal, and the perceptual evaluation of speech quality shows that the post processing method improves the score of the speech signal 0.55.