The rising of pore water pressure and decreasing of matric suction in the process of rainfall infiltration into slope is the main factor of slope failures .Numerical simulation method of critical slip field of slope considering rainfall process is proposed based on proposed unsaturated soil shear strength theory and water pressure field obtained by finite element analysis of saturated-unsaturated seepage. Thus

slope local safety factor and slope whole safety factor as well as the corresponding critical slip surfaces during infiltration are calculated conveniently and rapidly. This method has been applied to the stability analysis of two example slopes and the effect study of rainfall intensity

duration and strength parameter on slope stability. The results show that the critical slip field method could search for any shape of the most dangerous slip surface; and the safety factor is reasonable.