Because of differences in geological structure, litho logy and weathering degree of slope, there is double-decked exist in space composing of the slope, it is also called potential sliding surface. So, the seismic behavior of slope is hard to synchronize with each other. Upper layer of the slope will produce permanent displacement, and it will decrease the stability of the slope. Permanent displacement is increased by continuance seismic loading and even to lead slope failure. Thus it can be seen, permanent displacement is quite important to estimate the stability of slope and seismic design of reinforcement structures in earthquake zone. So, the paper based on the upper bound theorem to deduce the Formula for calculating the critical seismic acceleration of translational slide and rotational slide respectively. The formula for evaluation permanent landslide displacement under seismic loading has been created by considered the balance relation between seismic energy and mechanism dissipation energy. At last, an example is give to prove the theory.