Abstract: In this paper, we propose an energy-efficient algorithm for which weighs the diversity and multiplexing of MIMO technology. We design the signal propagation model of UWSNs, and realize an energy-efficient solution based on the tradeoff between multiplexing and diversity of MIMO. Finally, we conduct the simulation experiment by MATLAB, then compare the performance of three algorithms, which are MIMO full diversity, MIMO full multiplexing and MIMO based on the tradeoff diversity and multiplexing. The simulation results show that the proposed algorithm in guarantee the transmission reliability on the basis of reducing the energy consumption, and prolong the survival time, enhance the transmission rate and reduce the bit error rate of UWSNs.