Abstract: An underwater acoustic communication scheme using time delay and multiple frequency modulation technique is presented in this paper. It uses multiple frequency to fill in the CW narrow pulse′s chips for carrying m-bit information,and uses time delay between two CW narrow pulses filled with different frequencies to carry n-bit information. The receiver uses time delay estimation and frequency detector to decode. The chip interval is made long enough to overcome the multi-path effect,improve frequency band availability,and increase system reliability. Results of sea test show that,by using the time delay and multiple frequency modulation (TD-MFM) technique,the transmitting rate is sufficiently high and the system is robust.