Considering the matrix suction and seepage force of unsaturated clay slope, the limit equilibrium state is discussed according to the sliding surface over or under the water line. Then, the force and moment balance equations are established in the limit equilibrium state of the sliding slope respectively. The formulation of the limit equilibrium slice method is obtained for calculating the unsaturated clay slope stability under rainfall infiltration. The related parameters of the equation are obtained through test, parameter transformation and the position relationship of the sliding force. Then, the interaction force coefficients and the safety factor of the unsaturated clay slope are determined in the limit equilibrium state with numerical calculation method. The case study results show that the safety factor of the unsaturated clay slope is lower about 13.8% considering seepage force than without considering the seepage force, and the change rate of coefficient between slices considering seepage force is obviously larger than that of coefficient between slices without considering the seepage force. The instability time of slope is approximately the same when the rainfall intensity exceeds a threshold with the formation of slope runoff soon.