Based on the silt measure of reservoirs and ponds and the investigation ot all factors concerned, after the statistical analyses of all informations, this paper presents a mathematical erosion model used for the small watershed in the loess areas. The research results show: the prediction equation for annual soil loss in a small watershed which is based on the analyses of the leading factors has a higher precision. The equation objectively reflects the comprehensive roles between the concerned factors. It not only calculates, according to the given-data, both previous and present soil loss each year, but predicts the annual soil loss at a certain rainfall level in .the future. In the meantime, this paper more comprehensively makes quantitative analyses of soil erosion law in the loess hill area and indicates the interactions and relationships between the factors influencing soil loss. So it gives the reliable functions to establish a watershed management plan and to control soil erosion with the different efforts.The research results have been applied to the local areas and adopted by the foreign specialists of WEP/FAO evaluation mission.