By using the raster data of landuse and soil types,this paper builds a raster-based SCS model,which in turn enables the improved model to make a better consideration of the heterogeneity of watershed underlying surface and the heterogeneity of rainfall. In addition,this paper applies the new model in a real watershed,result from which shows that simulation accuracy can be significantly improved by the raster-based SCS model,compared with the original SCS model. Among the 44 rainfall events in total,the number of the rainfall with a runoff simulation accuracy of above 85% increases from 12 to 26. What's more,runoff distribution maps are obtained using the improved model,which help provide more accurate runoff information on the water management of watershed.