Abstract: By combination of survey data and field investigation of the Tianchi pumped storage power station dam, the distribution of altered rock was researched. By use of thin section identification, scanning electron microscope and X-ray diffraction analysis test, the altered rock of rock composition and chemical composition was analyzed. The geological causes of alteration and alteration mechanism was studied and the physical and mechanical parameters of altered rocks were obtained with laboratory experiments; altered rock riverbed toe slab foundation is numerically calculated and analyzed by using FLAC3D software in the different conditions and the different elevations. The results show that:on the left bank and right bank of the maximum depth of weathering is not as strong and the proposed excavation and toe slab foundation was in the bar in the middle of the mixed granite; the alteration of sand-like core presented in the range of dam foundation F₂₅ fault and around the fault zone is the main geologic factors of dam deformation and the adverse impact of the dam settlement; the numerical simulation of river altered rock toe slab foundation optimization showed that the riverbed of the 930~950 m elevation being as base surface is appropriate.