Abstract: Earthquake is an unfavorable factor in the slope stability problems. The effect of seismic action on the stability of three-dimensional (3D) soil slopes with the nonlinear criterion needs to be further studied. For 3D homogeneous soil slopes undergoing the horizontal earthquake, this study established the 3D rotational slip failure mechanisms (face-failure mechanism and base-failure mechanism) with the nonlinear strength criterion. The pseudo-static method was used to consider the work rate done by the horizontal seismic load, and the energy balance equations for two kinds of 3D slope sliding bodies were established. For a 3D soil slope with the known geometric parameters and the known strength parameters, the stability coefficient, the critical slip surface and the equivalent strength parameters for 3D slopes were obtained by using the numerical optimization method. The influences of the horizontal earthquake on the slope stability and the equivalent strength parameters were well analyzed. The slope stability charts were drawn to present the references for the practical engineering. The research results show that the stability of 3D soil slope decreases with the increase of the horizontal seismic action. Especially for the slope with a small inclination and a large width, the decrease degree of the slope stability is more significant. The horizontal seismic action has an obvious effect on the equivalent strength parameters only when the nonlinear parameter ratio c₀/σ₀ is large. In this situation, the equivalent internal friction angle will become bigger with an increase in the horizontal seismic action, and the equivalent cohesion will become smaller with an increase in the horizontal seismic action. The location of the critical slip surface of soil slope turns to be deepened as the horizontal seismic action becomes bigger. Besides, the 3D characteristic of the slope critical slip surface will become significant with the increase of the horizontal seismic action.