It is important to locate the slip surface in stability analysis of soil slope. The current method, circular slip surface method, results in a major error in the simulation analysis. In this paper, a method for seismic stability analysis of soil slope is proposed based on the theory of slip line field and dynamic finite element. Firstly, the stress field of soil slope is determined under seismic loading. Then, the method of slip line field is used to search the critical slip line under different conditions, and safety factor for every slip line is achieved. Finally, the seismic stability analysis of soil slope is quantified by the minimum mean safety factor. The proposed procedure is used to determine the critical slip lines and safety factors of an example slope at different moments. Seismic stability of the soil slope is evaluated by using the minimum mean safety factor. The study indicates that the shear band is composed of the slip lines with safety factors below 1.2. The shear band is consistent with the plastic zone of soil slope.