Abstract: This paper presents building principle and accomplishing approach of Assistant Decision System for Design and Decision Expert System of Rational Land Use(DDES),and discusses the means of expert knowledge expression and inference control strategies in the DDES. The field expert knowledge is divided into several grading levels and classifications in the system. And the frame-net data structures and productive rules are used to express special knowledge and experimental knowledge of geographic field expert so that the expression of knowledge is more concise and easy to understand. Meanwhile the indistinct inference mechanism is introduced in the inference tactics of knowledge so that the reasoning mechanism is assuredly realized. Furthermore, taking Urumqi river basin as a test region, the author expounds expression approach of expert knowledge for land rational use design and decision in the watershed,and probes the basic rules of expert knowledge expression as well as the organization model of expert knowledge in the system. Moreover, the author also discusses the design principle and organization approach of inference rules for inference engine of the system as well as inference algorithms of inference engine.