The auxiliary converter of the 350 km/h CEMUs adopts wireless parallel technology, which improves the power supply redundancy. The auxiliary converter was controlled with PQ droop control. The phase-locking of supply bus voltage was realized with three-phase synchronous coordinate transformation mode, grid-connected flow control was realized by means of active power controlling frequency of auxiliary converter and reactive power controlling output voltage amplitude of auxiliary converter. A real-vehicle test had been conducted. The test results show that the 350 km/h CEMUs auxiliary converter has a good grid-connected control effect with fast dynamic response and nice current sharing effect in the process of starting, switching, and load fluctuation, which meets the requirements of the EMUS operation.