Abstract: According to the aggravation status of water loss and soil erosion in the purple soil region, Hemingguan watershed of Nanbu county in Sichuan Province was selected as study region and a distributed model of runoff based on the natural storage model was developed in order to quantitatively analyze the degree of soil erosion and water loss. The model takes plot of land as calculated unit and the parameters were inputted for each plot, then the runoff of each plot was calculated based on the mechanism of the runoff, finally, the total runoff was calculated by recursion algorithm from the peak of watershed. The model can also evaluate the effect of variation of land use and support practice on runoff, and simulate the runoff process of rainfall in each plot of land. The model was finally validated in Hemingguan watershed and the results showed that the proposed model could reasonably simulate rainfall-runoff in small watershed. The research can offer the evaluation of regional soil and water conservation.