According to the "FAULT TREE ANALYSIS" method and "SYSTEM SAFELY THEORY", this document firstly establishes the KBS rules of an air-conditional diagnostic expert system on the base of production system. Furthermore it describes and normalizes the KBS rules based on the relation knowledge expression mode, then it realizes the knowledge deduction on the premise of making full use of the strong ability of data processing ability of RDBMS. Simultaneously, the uncertainly deduction system and the double direction deduction between cause and result are also realized.