Abstract: One of the main reasons for the reduction of irrigated land in China today is the aging and defaults of irrigation structures. In order to solve the problem. Investigation and diagnosis of the defaults of irrigation structures must be made so that effective counter measures can be taken accordingly. An irrigation structure defaults diagnosis expert system (ISDDES) was developed for this purpose. Based on the characteristics of irrigation structures and the principles of expert system, this paper discusses the function, environment, structure, knowledge representation, practical strategy, technical measure and knowledge refinement of the ISDDES and then points out the direction for improving the system.