The working principle of a loader hydraulic system was analyzed. On the basis of establishment of loader hydraulic system fault tree, modern testing techniques were used for loader hydraulic system operating parameters online monitoring, and PHP and MYSQL development technology were used to design loaders humancomputer interaction fault diagnosis expert system. Through the design of wireless network interface, the Socket transmission of monitoring system and expert system information was realized. When a failure occurs in the course of the loader, through the fault analysis of external performance and monitoring parameters online, the application of expert system can eliminate and solve problems quickly.