Taken an open pit mine's slope as the engineering background, based on the unified strength theory, different strength criterions were approximated by adjusting the two constitutive parameters of b and α, and a userdefined constitutive model was constructed, which was suitable for actual working conditions. After drawing up the improved strength reduction rule, the slope safety factor was computed by reducing the related strength parameters. Compared with the traditional strength reduction method based on the Mohr-Coulomb strength criterion, which ignoring the effect of intermediate principal stress, this improved strength reduction method overcomes the defect of calculated results being too conservative.