With the rapid development of the communication technology, the high quality of speech communication has become one of the main development trends. In order to solve the quality problem in the processing of communication, mixed excitation linear prediction speech coding algorithm (MELP) because of its higher compression rate and better security performance, is widely used in communication systems. In this paper pre-emphasis is used to improve the quality of the high frequency components in this algorithm. The speech enhancement of spectral subtraction is also applied in this speech coding algorithm to improve the ability of anti-noise. Adaptive difference pulse code improvement is also used in the quantization part of the speech algorithm to obtain the transmission rate that is much lower than before. Furthermore, the algorithm is transplanted into the digital signal processing system in order to validate the feasibility of this algorithm. At last, the speech quality will be proved by PESQ - MOS test and ...