In this article, we study the fault diagnostic method based on fuzzy logic for designing the fault diagnostic expert system of antiaircraft missile. Firstly, the designed idea, structure and working principle of the expert system (ES) are described briefly. Secondly, the method of complex knowledge expression of "production rule + frame + fault tree structure", repository establishment and an inexact reasoning model of operator are introduced in greater detail. Finally, the results of practical application show that this method is effective and feasible.