Abstract: The underwater acoustic channel is characterized as a time-dispersive rapidly fading channel, which in addition exhibits Doppler instabilities and limited bandwidth. To eliminate intersymbol interference causing by multipath propagation, spatial diversity equalization is the main technical means. The paper combines the passive phase conjugation and spatial processing to maximize the output array gain. It uses signal-to-noise-plus-interference to evaluate the quality of signals received at different channels. The amplitude of signal is weighted using Sigmoid function. Second order PLL can trace the phase variation caused by channel, so the signal can be accumulated in the same phase. The signals received at different channel need to be normalized. It adopts fractional-decision feedback diversity equalizer(FDFDE) and achieves diversity equalization by using different channel weighted coefficients. The simulation and lake trial data processing results show that,the optimized diversity receiving equalization algorithm can improve communication system's ability in tracking the change of underwater acoustic channel, offset the impact of multipath and noise and improve the performance of communication system. The performance of the communication receiving system is better than that of the equal gain combination. At the same time, the bit error rate(BER) reduces 1.8%.