Based on saturated-unsaturated seepage theory and unsaturated soil shear strength theory, a typical high cutting excavation slope in Jiangxi Liren-Longnan Section of Daqing-Guangzhou Expressway was selected, the model was built by the simulation software GeoStudio, and the shape and mechanism of slope instability of granite residual soil cutting excavation with rainfall were studied by the means of limit equilibrium method and finite element method of rainfall infiltration. The results showed that groundwater table rose from the bottom of slope and “small arc” instability easily happened in the bottom with rainfall; the increase of volumetric moisture content deceased the matric suction, and thereby deceased soil strength and safety factor of slope, and the decreases of matric suction and safety factor were synchronized.