Abstract: In attempt to make use of water and soil resource in the watershed of purple soil reasonably, we simulated the runoff from rainfall in the slope land of purple soil, providing scientific basis for simulating the no-point source pollution from farmland. The experimental plot situated at the middle of Sichuan Basin, the peculiar resource of purple soil and subtropic climate make it become the principal part of Sichuan agriculture.This paper，using Soil Conservation Service Curve Number method（SCS），calculated the volume of rainfall runoff in 2005, through calibrating the parameter Ia and parameter CN. it can received the suitable range of parameter, and the arverage error are respectively 6.46%,10.22% and 8.40%. It also show the feasibility that the application of SCS model to estimate the volume of rainfall runoff in sloping field of purple soil