The invention relates to an AC-DC-AC auxiliary converter of a motor train unit and a working process of the AC-DC-AC auxiliary converter. The auxiliary converter comprises a controller, a pre-charge circuit, a boosting transformer, an input filter circuit, a rectifier circuit, a discharge circuit, an inverter module, an output filter module and an output contactor, wherein the pre-charge circuit, the boosting transformer, the input filter circuit, the rectifier circuit, the discharge circuit, the inverter module, the output filter module and the output contactor are connected with the controller; when the auxiliary converter is started, the pre-charge circuit is charged; when charging voltage reaches a certain level, the pre-charge circuit is disconnected and the auxiliary converter works without resistance; when the auxiliary converter works normally, the auxiliary converter inputs three-phase AC and boosts the three-phase AC through the boosting transformer TR1; when input voltage is over a zero point, a PWM rectifier is started, output voltage reaches stable DC voltage through a unipolar four-quadrant double closed-loop control algorithm of the PWM rectifier; rated three-phase AC voltage is output through a voltage space vector SVPWM control mode of the inverter; and a load is provided with electricity through the output filter circuit and the output contactor. By the auxiliary converter and the working process thereof, the power supply reliability of the auxiliary converter can be improved.