Take a mountain side slope in Xiuyan County of Liaoning Province as an example, the potential sliding surface is found out with trenching, drilling and large-scale shear tests. 3D numerical simulation is made on the side slope with FLAC3D software to analyze the relationship between stress and strain in order to determine the area with large deformation and stress concentration; meanwhile by using strength reduction method to draw safety factor of the landslide and evaluate its stability. Comparing the data obtained in the deformation monitoring points arranged at the tensioning fractures on the back edge of the slope with the numerical simulation results, the reliability of the numerical simulation can be verified to provide the basis for the anti-sliding design of the side slope.