Based on factor analysis of soil erosion, a categorical model of estimating soil erosion based on GIS has been proposed. The key to this model is to set up the relationship between soil erosion and driven factors, such as rainfall, terrain, cleuch density, percentage of vegetation coverage, soil texture and so on. It so happened that Geographical Information Systems provide the functions to manage manipulate those datasets. Case study of Xihanshui watershed showed that the categorical model based on GIS has the advantage of quantitative estimation of soil erosion,can be used to monitor soil erosion dynamically.