The simulation of a high-order modulation LDPC code is carried out in the field of underwater acoustic channel for improving the spectral efficiency.The computation complexity can be reduced by using the simplification algorithm of soft information of the high-order modulation.Effects of the code length,times of iterations and other related factors on the performance of LDPC codes are analyzed and simulated.The results show that the high-order modulation LDPC codes can significantly improve the transmission performance.