Abstract: This paper presents the approaches and principles for building a Water and Soil Conservation Expert System (WSCES) in loess regions.The system can be used coding.The WSCES can be also used to store,update,inquire,search,analyse,process,display data and images, and to make map automatically.In addition,the system can be used to predict the soil erosion in loess regions precisely and quickly.Furthermore,several plans and decisions for comprehensively harnessing soil erosion can be obtained by consulting with the system,so that users can select a practical program from them for bringing the water under controllig and improving the soil.Moreover,the author makes a test research with the system on soil erosion in Yangdaogou gully of Wangjiagou Watershed in Lishi County, Shaanxi Province, and evaluates the results systematically.