Abstract: In order to overcome inter-symbol interference (ISI) caused by multipath propagation over UWA channels,reliability and validity of MQAM-OFDM system aimed at UWA communications over 15 kilometers is studied and simulated. The results show that its capability of overcoming co-channel interference,appropriate coding and interweave and modulation can be used in transmission with multiple rates. MQAM-OFDM systems have good prospects in UWA communications.