The dynamic stability of slopes is influenced by the horizontal and vertical earthquake forces. The traditional method of calculating the seismic permanent displacement does not take enough consideration about the vertical seismic effect. However, considering the combined effect of horizontal and vertical accelerations is more accord with the engineering practice. Based on the upper bound limit analysis and the model of Newmark's rigid-plastic block, an approach for evaluating the permanent displacement of slopes based on the actual horizontal and vertical earthquake acceleration time-history curves is proposed. Three types of engineering slopes are studied to discuss the impact of two typical earthquake ground motions on seismic permanent displacement. The results show that: this method is compatible with the previous method without considering the vertical acceleration; the combined effect of horizontal and vertical accelerations is different for various earthquake records, and the influence of vertical acceleration on the permanent slope displacement cannot be ignored.