Abstract: The partial factor method is widely used in the design of structural engineering and tends to be developed in the design of geotechnical engineering. The optimum values of partial factors used in the stability analysis of soil slopes are discussed. First, the traditional slice method, the response surface method and the concept of design space are combined to calibrate the partial factors. Then, several groups of partial factors are used to design the soil slopes with variable shear strength parameters and geometrical shapes. The actual reliability levels of these designs are checked and used to optimize the selection of partial factors. The results demonstrate that: (1) The coefficient of variation strongly affects the calibration of partial factors; (2) The decrease of fractile ratios can reduce the variation of the calibrated partial factors and the real reliability levels of soil slopes designed by the partial factor method; (3) Instead of the design checking point, a point with certain degree in the design location is recommended for the calibration of partial factors; (4) When the target reliability index is 3.2 and the fractile ratio is used as 0.1, the partial factors γ_c=1.30 and γ_f = 1.15 are recommended in the design of soil slopes. (5) Using the proposed method, the real reliability levels of the soil slopes designed by the factor of safety method can be evaluated.