Rainwater infiltration into the ground is liable for the rising of transient pore water pressure during rainfall. Due to the significant role of infiltration in controlling slope failure, this study aims to investigate the factors affecting the rainwater infiltration during extreme rainfall. Parametric studies on the Green-Ampt infiltration model were performed to evaluate the effect of the parameters such as soil type, moisture-suction and slope angle. In this study, a new parameter (Mψ) was introduced as moisture-suction as the product of the suction head at the wetting front, and volumetric water content deficit. Rainfall condition has been collected from three typhoons attacking Taiwan in July 2006. The results showed that the increase in saturated hydraulic conductivity and moisture suction parameter could increase the infiltration rate. The infiltration decreases with the increase of the slope angle of the surface. For slope made of high permeability soil, the significant variation in moisture-suction has changed the infiltration rate considerably compared with the slope covered with the lower permeability material. This study proposes a new graphical aid to represent the basic Green-Ampt equation for the sloping of various gradients. Comparing the infiltration for sloping surface and horizontal surface, the reduction of infiltration rate is given by ksat (1 – cos β).