The research is funded by Department of Science and Technology fund project titled Ecological Environment Subject Studying for National Major Engineering. Supported by GIS(Geographical Information System) and remote sensing technology, it has carried on computer simulation of chemical runoff in the study area of Xiaojiang Basin, County, Chongqing City, using SWAT(Soil and Water Assessment Tool) model developed by ARS(Agricultural Research Service) of USDA(United States Department of Agriculture).The main contents and results are as follows:1. The computer simulation of chemical runoff using mathematical models in China.Here, the mathematical models are physical models. By GIS and remote sensing technology, the simulation is realized using SWAT model. The basin area is large reaching 1005km~2. The simulated values by year and moon of many nutrients such as N and P are obtained in Xiaojiang watershed and its 52 sub-basins. Besides, the simulation of chemical runoff of Xiaojiang Basin is carried on based on the land use in 2001 and in 2005. Also the results are analyzed from time (annual and moon) and space respectively.2. The extraction of the necessary relevant parameters in SWAT model.The data needed by the research are collected from relevant materials and field sampling, including the soil saturation hydraulic conductivity, soil bulk density, various kinds of chemical parameters of soil layer(such as organic nitrogen, organic phosphorus and soluble phosphorus), the transformation of organic carbon content of soil layer and the land use in 2001 etc.. The relevant parameters are extracted by GIS which makes it convenient.3 . The analysis of the relation between land use and chemical runoff.12 representative sub-basins, in which two interim simulated values in 2001 and 2005 varied significantly, are chosen. Then the relation between the land use and simulation value in those sub-basins is analyzed. It offers meaningful reference for agriculture sustainable development and comprehensive administration and planning of basin, and also gives technical supports foragriculture BMPs(Best Management Practices).