Abstract: According to the dynamic balance condition of the soil particle on the sloping land,the critical shear stress of the fine silt on the sloping land is deduced. Based on the surplus transport capacity of the overland flow,a calculation formula of the soil erosion of the overland flow is obtained. With the calculation formula of the rainfall splash in the reference four,a mathematical model with clear conception and definite process and complete structure of sediment yield from watershed is developed. Sediment yield depends on the comparison relation of the transport capacity of flow and supply sediment. If transport capacity of flow less than supply sediment,the seiment yield is equal to the transport capacity of flow. Otherwise,the soil will be eroded by the overland flow,and the sediment yield is addtion of the supply sediment and runoff erosion. Therefor, the sediment yield from sloping land and watershed can be calculated. The model is verified with measured data of Chabagou catchment and the results are satisfactory.