With the mountains behind the Liu’s as the object of study，and by utilizing FLAC3D software based on finite difference method, a three-dimensional numerical model of the slopes has thus been established. By adopting the Mohr-Coulomb criterion as well as the shear strength reduction theory, a stability analysis has been carried out to simulate the stress state of the slopes and find out the mechanism of the slope instability and failure patterns, followed by an analysis of the major affecting factors of the slope instability, thus obtaining the safety factor of the slopes. With the results of the numerical simulation combined with the field investigation results, some reasonable and effective measures for the slope reinforcement has thus been put forward.