Abstract: This paper discusses the meaning representation of fuzzy knowledge and the basic rules of fuzzy reasoning,and presents the design of the Fuzzy Diagnosis Expert System of Equipment (FDESE).In the FDESE,the fuzzy knowledge used by the diagnosis expert are divided into two parts: factual knowledge and diagnostic knowledge which adopt two-tuple and rule representation methods respectively.In the fuzzy reasoning mechanism of FDESE,we put forward the multi-class forward- backward fuzzy reasoning approach to simulate the reasoning process of expert and design a set of propagating function to calculate the uncertainty of the conclusion.