The invention provides an auxiliary converter which comprises an input filtering unit, a pre-charging unit, a pre-stage boosting unit, a medium frequency isolation DC/DC converter, a three-phase inverter unit, an output filtering unit and an external output control unit; the input end of the input filtering unit is connected with a high-voltage DC bus, and the input filtering unit is used for filtering DC inputted harmonic wave and common-mode interference; the pre-charging unit is used for pre-charging a DC support capacitor; the pre-stage boosting unit is connected with the pre-charging unit and is used for outputting expected DC voltage; the medium frequency isolation DC/DC converter is used for performing medium frequency inverting on the DC voltage outputted by the pre-stage boosting unit, square-wave voltage of the medium frequency is generated, isolation voltage transformation is performed on the square-wave voltage of the medium frequency, and the square-wave voltage is rectified as middle DC voltage; the three-phase inverter unit is used for inverting the middle DC voltage into three-phase power frequency alternating current; the output filtering unit is used for filtering the three-phase power frequency alternating current; and the external output control unit is used for controlling and outputting the filtered three-phase power frequency alternating current to external load.