Abstract: Due to little researches on dynamic stability of soil slopes by using the numerical manifold method (NMM), NMM is adopted to analyze such a problem. The viscous damping is introduced to dissipate the energy of the system. Through a few numerical examples, the effectiveness of NMM to solve the permanent displacement of a slope is verified. Using the stress field of the slope under dynamic seismic time history, the time history of safety coefficient is obtained, and the most dangerous sliding surface of the slope under seismic time history is searched. The most dangerous sliding surface model is introduced into NMM to obtain the permanent displacement. The results of NMM are compared with those of the Newmark method, and they are similar. This study shows that NMM can effectively slove the dynamic stability problem and simulate the permanent displacement of soil slopes.