Rock mass integrity of dam right abutment slope of so me hydro electric station is destroyed with Fault f1 developed, so stability pro blem is raised. The paper studies the slope stability using FLAC -3D. The result s show that tensile stress zones and buckling deformations appear in hanged wal l rockmass of fault during excavation. Hanged wall rockmass will be lost stabili ty as large deformation slipping along fault happens after excavation finished b ecause the lower part of fault will be exposed mostly. The safety factor determi ned by strength reduction method is less than 1.0, and slipping - failure zone delimited by localization band can be exhibited. The infl uence of seepage on slope is discussed. At last, the paper points out that the e xtension of Fault f1 is critical to slope stability and should be further explor ed and researched.