The left dam section of Xitou Reservoir was built on the foundation of completely weathered granite, its poor physical and mechanical performance may not meet the needs of the project. Combined with the results of in-situ test and indoor geotechnical test of dam foundation rock mass, the physical and mechanical properties and variation law of the completely weathered granite were studied based on the simplified calculation model of anti-sliding stability of the dam body, and the anti-sliding stability of the dam body was calculated to ascertain the application feasibility of completely weathered granite to the dam foundation of masonry gravity dam. The results show that the completely weathered granite in the left dam section is a continuous and gradual geological body from the surface to the underground, the weathering becomes worse and its physical and mechanical properties deteriorates even more from the underground to the surface. The bearing capacity of completely weathered granite foundation in natural state is inversely proportional to its weathering degree, and is significantly lower than that in saturated state, its reduction range is 55.4%-70.4%. The shear strength of completely weathered granite in saturated state is much lower than that in natural state. The reduction range of internal cohesion is 53.9%-75.4%, and that of internal friction angle is 83.1%-89.3%. The safety factor of anti-sliding stability of completely weathered granite dam foundation is relatively low in natural state, in order to improve the safety of the project, reinforcement treatment should be carried out. The safety factor of anti-sliding stability is extremely low in saturated state, which will lead to dam instability. In this case, completely weathered granite is not fit for the role of dam foundation and should be completely removed.