The strength reduction method is used widely in design of slope stability as a common method for calculating safety factor. This paper introduces the basic principle and computational procedure of the strength reduction method ； programs the calculation program in FISH language built in FLAC3D ； establishes numerical slope models under different factors and levels ； and calculates out the safety factors of slope stability under different working conditions using the strength reduction method. Finally, the relationship between safety factor and impact factors is obtained using grey correlation analysis. The results indicate that the initial stress field is the maximum one of all impact factors on simple isotropic slope, the intensity parameters and Poisson ratio have a bigger impact than geometric parameters, and the deformation modulus are the smallest.