Abstract: To further improve the reliability and band efficiency of the underwater acoustic communication (UWC), this paper offers a polar code construction algorithm based on the Monte Carlo method with two improvements for polar coded modulation (PCM) communication needs and the underwater acoustic (UWA) channel characteristics to match the underwater acoustic polarized channel of coded modulation. These optimizations are optimizing Monte Carlo statistics parameters based on the dynamical cognitive of the channel, and jointing decoding and decision feedback channel estimation. The proposed polar code construction method is then verified by establishing a two-step PCM UWC application mechanism. Through simulation, the performance of polar code construction methods before and after improvement, and the robustness to time-varying channels are then compared and analyzed, as well as the performance of polar code modulated UWC system with joint bit-interleaved coded modulation (BICM) and combination with multilevel coded modulation (MLCM) under different mapping rules. Also, it is compared with the low density parity check (LDPC) coded modulation system. The lake experiment demonstrates that the PCM UWC scheme suggested in this paper effectively ensures the reliable transmission of information in the shallow water channel, achieves error-free transmission at a signal-to-noise ratio of about 14 dB and a communication distance of about 1 km, and also has better error correction than the LDPC coded modulation system under the same circumstances.