In order to solve the fault diagnosis not accurate problem in traction system grounding protection test, research and analysis were carried out for system and components, which pointed out that the reason was the improper selection of IGBT driving device in traction power unit. By adjusting the parameters of capacitance and resistance of short circuit protection circuit of IGBT drive plate, the problems were solved. Through the research and analysis of the system and the optimization design of the IGBT drive circuit, the fault could be located accurately, which was helpful for the maintenance of the traction system of the EMUs.