Granite residual soil is widely distributed in southern China, and it has many poor physical and mechanical properties. Slopes of this type of soil are prone to deformation and failure under the action of rainfall. Therefore, it is of great significance to study the rainfall infiltration mechanism in granite residual soil slopes. On the basis of Green-Ampt model, the initial moisture content, underground water level and unsaturated characteristics of the soil are comprehensively considered. An infiltration model suitable for different rainfall conditions is established, and verified by comparison between numerical simulations and other models. Then, the proposed model is used to analyze granite residual soil slopes under three typical rainfall conditions. The results show that the initial water content has little effect on the migration rate of the wetting front when it is far from groundwater level, whereas the migration rate increases gradually when the wetting front is near groundwater level, presenting an exponential trend. In the same rainfall time, under the condition that the rainfall intensity is slightly greater than the saturated permeability coefficient of soil, the wetting front migration depth is the largest and the slope stability factor decreases the most.