Abstract: Due to the characteristics of large reserves, loose structure and low strength, an inner dumping site in the open pit coal mine is prone to erosion or overall instability under the rainfall conditions. The analysis of rainfall infiltration is quite important to estimate the stability of a dump. The inner dump of the Yuanbaoshan open pit coal mine is taken as the prototype, the physical model test is carried out according to the similar theory to study the characteristics of rainfall infiltration, and the classic Green-Ampt infiltration model is improved. The results show that during the process of rainwater infiltration, with the downward migration of the wetting peak, the erosion of the slope surface starts from the splash groove, gradually transits to runoff erosion, and finally appears as traceable erosion damage, showing the characteristics of shallow landslide parallel to the slope surface. The existence of unsaturated wetting layer above the wetting peak leads to the inaccurate calculation results if the classical Green-Ampt model is used. Based on consideration of the thickness of the infiltration layer and cumulative infiltration amount, an improved Green-Ampt model is derived. The improved Green-Ampt model is a piecewise function, which can reflect the actual characteristics of the transition from unsaturated to saturated slope caused by rainfall infiltration. The validation results show that the prediction accuracy of infiltration depth and cumulative infiltration volume is significantly improved. Because of the erosion damage and the difference in the initial water content, the prediction accuracy of the improved model decreases for the later stage of the rainfall, but it is of great significance for the stability analysis of an open pit coal mine dump at the initial stage of rainfall. The calculation result of the improved model is closer to the measured data.The improved infiltration model can provide an important reference for the studies of rainfall infiltration and slope stability of open-pit coal mine inner dumping sites.