The invention provides a vehicle working condition online monitoring system for a rail transit, and belongs to the technical field of rail transits. The monitoring system provided by the invention comprises a vehicle working condition data acquisition module, a composite sensor, a vehicle-mounted working condition information preprocessor, a driver information display system, a regional information processor, a ground center information processor and a network communication transmission system; and the fault region of a vehicle can be divided into a primary level, a regional level and a central level, and the state data can be sent to the ground through the network communication transmission system, so that the fault of the primary level is analyzed and processed through the correspondingvehicle-mounted working condition information preprocessor, the fault of the regional level is analyzed and processed through the regional information processor of the corresponding ground, and the fault of the central level is analyzed and processed through the ground center information processor of the corresponding ground. Therefore, different fault types can be timely and effectively solved at the most proper places, networking and intelligence of the monitoring system are realized, and running reliability and safety of the rail transit can be finally improved.