Abstract: This paper presents basic approaches and principles for how to express knowledge and to organize the knowledge base in Geographical Expert System(GES),and taking Water and Soil Conservation Expert System (WSCES) as an example, the author expounds the methods of classification and gradation for Water and Soil Conservation Expert Knowledge (WSCEK), and discusses the basic rules of knowledge expression in the WSCES and the organization patterns of expert knowledge in the knowledge base. In the mechanism of knowledge capture the system provides the mutual interface for taking supplemental edit of knowledge as a primary and understanding of natural language as a complement between man and system. Therefore, the input process of expert knowledge is simplified greatly, and working efficiency of thesystem is promoted to a high degree.

Key words: expression of knowledge, Geographical Expert System, organization of knowledge base