Abstract: Aiming at the problem of high decoding complexity and error floor of LDPC code in underwater acoustic communication system, a polar coded underwater communication system is designed, and a base symmetric spread channel polarization algorithm is proposed for underwater multipath channel polarization. By introducing high order base of PW, modified PW solved the problems of poor robustness and channel state dependence of polar code. Theoretical analysis and simulation results show that the modified PW method has stronger robustness in channel polarization than traditional polarization method. The proposed method is applied to OFDM underwater acoustic communication system, the designed Polar-OFDM system improves the performance in terms of the bite error rate (BER) and decoding complexity sufficiently in comparison with LDPC, and without error floor.