Abstract: At medium or long distance(>10 km)underwater acoustic speech communication,bit rate is constrained by the complicated,time varying channel and limited bandwidth of communication system.The bit rate of speech coding is required to be as low as possible.The large time delay of underwater acoustic wave propagation can be used for low bit rate speech coding.After investigating the Mixed Excitation Linear Prediction(MELP)standard and taking account of the auditory perceptual features,a variable and adjustable bit rate speech codec algorithm has been proposed, whose average bit rate is about 600 bps.The average Perceptual Evaluation of Speech Quality Mean Opinion Score (PESQ MOS)of synthesized speeches is about 2.8.It has been proved by the computer simulation and sea trial that the performance of the proposed algorithm is well and robust when bit error rate is no more than 10^-3.The synthesized speech is vivid and intelligible,and keeps main individual characteristics of speakers.