Abstract: Shear strength of discontinuities is an important parameter for landslide engineering. The back analysis is a low cost and effective method to determine shear strength of instability slope. This paper focuses on how to determine the shear strength of instability slope at Xibaipo Memorial Site using back analysis and laboratory test respectively. The instability slope has a single bedding slip. The results show that the shear strength of discontinuities of back analysis is similar to that of laboratory test. The back analysis can be used to evaluate the stability of bilateral slopes.