Abstract: This paper introduces a web-based remote fault diagnosis expert system of hydroelectric generation sets with WebService and ActiveX technologies. The system is divided into three parts: power plants, diagnosis center and experts. The WebService is adopted to capture, analyze and process data, and the ActiveX is used to present data and provide interaction with users. Comparing with a classical client-side application, the system is completely web-based and introduces the method separating data capture, analysis and representation for cross-platform calling data and analysis algorithm. That greatly improves the reusability and maintainability of the system.