Abstract: The stability of the south high slope of Xiaolangdi intake towers is a key problem to the construction of the project.There exist complicated rock formations,clay intercalation and large joints in this high slope.It makes the problem very complicated and then very difficult to solve numerically.In this paper,the stability analysis was made in consideration of fault F 238 ,main clay intercalation and dip angles and dip directions of rock joints.The simulation of the various construction stages and construction measures taken in the construction were made by the use of three dimensional FEM.According to the stress field,the rigid limit equilibrium analysis method was applied to 23 sections in three possible sliding ways. 20000 potential sliding surfaces were examined to every possible sliding way of each section.The minimum safety factor and corresponding potential sliding surface were given to each section.The results had been applied to the support design of the high slope and verified in the construction.