For the old landslide of the expansion project of som e hydropower station, the deformation failure mechanism and stability of the slo pe were respectively studied through the finite element method and the ultimate equilibrium method, and the two times local sliding of the old landslide during excavation testified the results of previous stability analysis. Based on monito ring data analysis and mechanism investigation, some synthetically reasonable countermeasures, such as, filling slope toe and installing slope drainage etc., were taken in time, and safety in construction was guaranteed. The method of stability analysis and experience of engineering reinforcement of the old landslide can be useful for other similar slopes. 