Earthquake is an important inducement factor of the landslide hazard. For three-dimensional (3D) rock slopes, this study utilized the tangential method to establish the 3D failure mechanism with the Hoek-Brown criterion. The 3D limit analysis method considering the earthquake was conducted by using the pseudo-static approach. Investigations were made to assess the effects of earthquake on the 3D rock slope stability. The results indicate that the slope stability will reduce as the earthquake becomes bigger, and the effect degree will be more significant for slopes with smaller inclination or width constraint. The equivalent Mohr-Coulomb parameters are closely related to the earthquake, and the influence degree will decrease with the increasing inclination. As the earthquake becomes larger, the critical slip surface seems to be shallower and its 3D effect becomes weaker.