Abstract: The fault diagnosis system for rail transit vehicle is an important part of vehicle network control system. A reliable and stable fault diagnosis system will provide effective support for the data security and analysis of rail transit vehicles. The design of real-time fault diagnosis system was based on VxWorks and Linux operating system. By acquiring, processing, communicating the data through MVB bus, the system can discover, record, analysis and report the fault and the running state to monitor and diagnosis the vehicle subsystems. The system has a good real-time performance and operational, is equipped with independent nonvolatile data storage module, a "black box" with IP54 protection grade, in the case of unpredictable collision, fire, can still maintain data integrity. The functions of maintenance, monitoring and analysis of fault and running data are implemented by communicating with the PC via Ethernet connection, and also through the WLAN based wireless vehicle transmission module, it is implemented the automatic downloading of the running state data and the fault data. Data can be real-time monitored, also can be displayed and analyzed in the form of a curve or table view. The application shows that the fault diagnosis system can record and analyze the vehicle running data and the failure data well, and has a strong operability and practicability.