An analysis method for slope safety factor through c - cp reduction algorithm by finite elements is presented. When the system reaches instability, the numerical non-convergence occurs simultaneously. The safety factor is then obtained by c - ψ reduction algorithm. The same time, the critical failure surface is found automatically. The traditional limit equilibrium method can't get the safety factor and failure surface of jointed rock slope. Strength Reduction FEM (SRFEM) presents a powerful alternative approach for slope stability analysis, especially to jointed rock slope. This paper analyzes the precision and error caused by different soil yield criterions、FEM itself、slope height、slope angle´cohesion and friction angle thoroughly. Through a series of case studies, the results show average error of safety factor between Strength Reduction FEM and traditional limit equilibrium method (Bishop simplified method) is 3 %～8 %，the error between SRFEM and Spencer's method is 1 %～4 % . The applicability of the proposed method was clearly exhibited.