A landslide lies in?the upstream of dam site at the right bank of Yalong River. Once the landslide is damaged. It will have an import an impact on the local residents and safe operation of the preparing hydropower station.This paper based on the field engineering geological mapping, drilling and laboratory test, the stability is analyzed by the numerical simulation using three-dimensional finite difference software(FLAC3D).It analyzes the basic characteristics, formation mechanism and stability of the landslide. The result shows that the landslide is a oversize rock?landslide. Its formation and evolution has passed three stages: shear peristalsis, flexural slumping?and transfixion failure. The landslide is stable under natural state and lacked of stability in ,but secondary sliding may occur on local surface.