This paper presents a procedure for calculating the safety factor for an unsaturated and anisotropic slope suffering from rainfall infiltration. The process of infiltration into a slope due to rainfall and its effect on soil slope behavior were examined using a one-dimensional finite element flow deformation coupled analysis program. The modified Mohr-Coulomb failure criterion was adopted for unsaturated soil strength considering variations in strength due to the presence of matric suction. A safety factor was calculated based on the smoothed stress field obtained from finite element analysis; and an optimization technique was used to search for a critical slip surface. Some numerical examples are analyzed. The results illustrate a typical process of infiltration into unsaturated soil slopes and the effect of hydraulic conductivity on slope stability during reinfall-induced infiltration. And the analytical data indicate that the existence of the cranny has a distinct effect on anisotropic slope stability. The effect of cranny should be considered in analyzing stability of slope infiltrating by rainfall.