Abstract: Speech coding at bit rate 4 kb/s is an important problem in the field of speech signal processing, it's also the next goal for ITU_T making standard. A speech coding algorithm implemented by mixed excitation is provided in the article, whose voice and unvoice are excited by harmonics and codebook respective, The vocoder can output good quality speech with short delay (the length of frame is 10 ms,sample rate is 8 kHz). The principle of the algorithm is analyzed in the article, a result of test is also provided.