Abstract: It is difficult to diagnose automobile engine’s abnormal sound breakdown because it is fuzzy. For diagnosing the breakdown rapidly and accurately, this paper deals with the fuzzy diagnosis method on automobile engines abormal sound based on the fuzzy mathmatic theory and puts forward mathmatic modeling of fuzzy diagnosis, method of building fuzzy relation rectangular array and diagnosis processing of fuzzy reasoning. By applying these techniques, the automobile breakdown diagnosis expert system has high accuracy and reliability on diagnosing engines abnormal sound. It has been proved in the process of repairing on some types of automobile engines. 