Abstract: With the rapid increase of urban rail transit operation mileage, the growth trend of vehicle electric component fault rate also appeared, many rail transit vehicles have entered the stage of unwheeling repair or overhaul. The auxiliary inverter system is a major part in metro vehicle electric equipment, and the key component of the system is auxiliary inverter. Based on its working principle, the design of auxiliary inverter off-line diagnose system for urban rail transit vehicles is introduced from the angles of scientific demonstration and computer verification The design schemes of DC power supply system module, load module, signal conversion mode and control system module are emphasized. Finally, the off-line diagnose system is simulated and the result shows that the system has good performance, and is currently applied to the fault repair, unwheeling repair and overhaul of auxiliary inverter.