Based on the three-dimensional finite element transient fluid structure coupling calculation of Yanshan Reservoir Dam, the rainfall infiltration process of dam slope under heavy rainfall was simulated. On the basis of rainfall infiltration simulation results, combined with the particle characteristics analysis of dam slope cushion material and field survey results, the rainfall erosion process of dam slope cushion material was studied. The results show that: rainfall infiltration is the main cause of the erosion of the cushion material of the downstream dam slope, and the area where the hydraulic gradient is greater than the minimum critical hydraulic gradient of the cushion material is the main area of erosion; in the process of rainfall infiltration, with the continuous loss of fine particles, the particle gradation of cushion materials tends to deteriorate, and the voids between precast blocks and cushion materials appear; in order to ensure the safety of downstream dam slope, it is suggested to adjust and design the particle gradation of cushion material, improve the critical hydraulic gradient value of cushion material, and enhance the drainage capacity of dam slope by adjusting precast block and setting drainage ditch.