The traction power supply system of electric multiple unit (EMU) adopts 25 kV single-phase transformer to draw power from the traction catenary. The disconnection of pantograph-catenary during the locomotive running operation, nonlinear characteristics of power devices, and the asymmetry of hardware circuit and control system can all cause the DC components to inject into the vehicle traction transformer through the electrical circuit, and result in the DC magnetic bias of transformer. By applying the proportional-integral-resonance (PIR) control to the current inner loop controller, the stable track of the sinusoidal signal and the suppression of the DC components are realized. The simulation and the hardware-in-the-loop simulation results show the effectiveness of the control strategy.