Abstract: Rainfall erosion and its influence factors have been analysed in this paper.The formula of impact force produced by raindrop is deduced with the Newton's second law of motion based on the law of rainfall erosion on upland of ridge-hill in Loess Plateau regions.1t is assumed that splash rate of raindrop correfates with erosion force of raindrop to be linear,and a splash rate formula of raindrop is developed.It is verified with measured data of runoff plat of Qiaogou catchment and the results are satisfactory.