As the rapid growth in computer technology, agricultural informatization is becoming a powerful tool that overcomes the weakness of conventional agriculture, and the agricultural expert system will be the impulsive factor as well. The expert system of diagnosis for orchard pests and diseases developed in this paper is based on the research in expert system, production rule reasoning, Web technology etc.,integrating the characteristic of pomology. This system combines the knowledge framework of orchard pests and diseases diagnosis with expert system technology, enabling the users to diagnose orchard pests and diseases, and acquire the related information and controlstrategies of these pests and diseaseswhich has highly comprehensiveness and value in use. Using this system, the level of prevention from diseases will be improved, so will the agricultural technology.It can also prevent or reduce disaster losses, and then improve the economic, ecological and social benefits.