Aiming at the problem of solid calcium decomposition under seepage erosion of concrete dams, based on the analysis model of cement-based material advection-diffusion-driven leaching model, the calculation method for concrete dam leaching quality was proposed. Taking the RCC gravity dam of the Shimantan Reservoir as an example, the distribution of the leached quality of different dam sections was studied. The calculation results show that the simulation results are in good agreement with the monitoring data, which verifies the correctness of the method. The leached parts of the dam cementitious materials are concentrated at the bottom of the concrete face slabs, the upstream side of the key-wall and the connection part of the curtain and the key-wall. The existence of the drainage pipes can affect the leaching rate of the cementitious materials. After 100 years of leaching duration, the mass percents of the cementitious materials of the curtain, key-wall, panel and dam body are 5. 5%, 5. 6%, 4. 5% and 1. 2% respectively, which shows that the advection-diffusion-driven leaching has a great impact on the curtain and the key-wall. In the operation of the project, it is necessary to strengthen the leaching monitoring of curtain and key-wall.