The Newmark permanent displacement, an important index for evaluating the stability of slopes in earthquakes, is extensively applied to seismic slope hazard assessment. However, because the critical acceleration is assumed to be constant and the variations of shear strength parameters on sliding surfaces are not considered in the traditional Newmark permanent displacement method, the permanent displacements of a slope are often underestimated. To solve this problem, this paper analyzes the change process of shear strength parameters on a sliding surface in detail based on geotechnical structure theory. A method to calculate the dynamic critical acceleration in which the dynamic change of cohesion is simulated by the Monte Carlo method on the basis of a certain probability distribution type of cohesion is presented. Sample calculations reveal the dynamic change processes of cohesion and critical acceleration of the slope sliding block in earthquake, and a new permanent displacement result is obtained. The displacement results conform to the normal displacement value of the seismic slope, and the implementation processes of dynamic cohesion and critical acceleration presented in this paper correspond to the seismic time history. Dynamic changes in shear strength parameters are solved to a certain extent, and the displacement problem encountered in the traditional Newmark permanent displacement calculation is addressed.