The invention discloses a fault reasoning method of an expert system based on symptom, which generates a binary rule set according to a propositional logic expression. A binary rule is a production expression, which is triggered by one or two triggering inputs to carry out logical operation and output the intermediate state as another triggering input of the binary rule or the conclusion of the rule; according to the data symptom table, matching to obtain the symptom of the system state data, each symptom represents a kind of state of the data; the symptoms of the state data are matched with the binary rule, and when the binary rule is satisfied, the intermediate state or the fault conclusion is generated according to the binary rule. The invention defines a new representation method of expert system knowledge based on symptom mode, and designs a corresponding inference engine rule pattern matching method. Compared with the existing reasoning mechanism of expert system, this inventionuses bitmap to represent the internal state of reasoning machine and adopts mode. The matching method of rule reverse index and the reasoning mechanism are simpler and more efficient.