Abstract: The linear strength criterion is widely used to describe the shear strength of geotechnical materials, but studies have shown that almost all strength envelopes of geotechnical materials have nonlinear characteristics. In addition, slope instability is often influenced by adverse natural factors, especially in areas of frequent seismic activity. Therefore, based on the upper bound method of limit analysis, a seismic stability analysis method of three-dimensional (3D) soil slopes under nonlinear strength criterion is proposed in this paper. The multi-tangent method is used to approximate the envelope of the Power-Law strength criterion to account for the nonlinearity of the soil strength. And a new 3D multi-cone failure mechanism is introduced to describe the nonlinear Power-Law criterion. The modified pseudo-dynamic method is used to characterize the seismic load, which does not violate the zero-boundary condition and considers the damping properties of geomaterials. According to the work- energy balance equation, two measures for evaluating slope stability are derived, and the optimal solution is found by combining genetic algorithm and sequential quadratic programming algorithm. By comparing with the existing literature, the effectiveness and accuracy of the proposed method are verified. Further parametric studies reveal the influence of soil strength nonlinearity, seismic action and 3D geometric characteristics of soil on slope stability.