In this paper, according to the characteristics of maize disease knowledge, OWL DL language was used to build maize diseases ontology, and the reasoning rule of maize diseases was defined by using the expressive ability of SWRL rule language.The author introduced several realizable reasoning functions,and achieved the diagnostic reasoning of maize disease knowledge by Jess inference engine.The results indicated that constructing the maize diseases ontology,and introducing SWRL rule into maize disease ontology provided an effective way for the construction of high-intelligent, shareable and reused maize disease knowledge database and diagnostic rule database.