To overcome the inefficiency that not less than two landslide sections are required in the back analysis of shear strength parameters based on the traditional slope stability analysis method, a new method is developed for back analyzing soil strength parameters based on the probability reliability theory, by combining the Excel spreadsheet method and the upper bound limit analysis method. The proposed method is applied to back analyze Zhuequedong landslide in Hunan province. The results indicate that the shear strength parameters of slip surface obtained by the proposed method are in good agreement with the measured data of the engineering site. The applicability of the new analytical method is verified. Based on this method, the inverse problem of shear strength parameters of a single slip surface can be resolved by using the measured data of only one landslide section, showing the effectiveness of the proposed method.