Abstract: In traditional fish disease diagnosis expert system, the representation of knowledge was not accurate and reasoning efficiency was low, and knowledge sharing among different expert systems was very difficult. In order to solve these problems, a fish disease diagnosis expert system based on ontology was established. The modeling of fish disease diagnosis knowledge ontology was built by improving the six step冶, and then the fish disease diagnosis expert system was producted. Fish disease diagnosis expert system established in this paper could greatly improve the accuracy of the knowledges representation, the efficiency of reasoning, and can realize the sharing of fish disease diagnosis knowledge.