Abstract: In order to reduce or avoid the losses of slope failures caused by earthquake, it is very necessary to study the slope stability and support issues under seismic forces. To calculate the lateral forces of the slope under the earthquake, the seismic forces as a horizontal force and the direction of the force point to the outside of the slope. With the assumptions that the slide surface is straight line, the lateral forces of slope are deduced by limit equilibrium method, and a general formula of the lateral forces is established. The lateral forces can be used to the design of retaining structures.