The auxiliary power supply system is an important part of the Electric Multiple Unit (EMU) technology, which ensures the safe and stable operation of the EMU for a long time. This paper centers around a testing equipment for grid-forming converters onboard railway vehicle, which firstly introduces the auxiliary power supply system adopted by different models in China. This test system adopts the Grid-forming control strategy and is composed of all silicon carbide (SiC) devices. The excellent characteristics of SiC devices can make the system lighter, more efficient and more high frequency. Grid-forming control is characterized by voltage source properties, which can improve the active support capability of the converter. Droop control and virtual synchronous generator (VSG) control are two typical grid-forming control methods. Then the principles and characteristics of the two control methods are introduced respectively. Finally, the three functions of the test system are introduced. The ability of the system to act as a load and as an auxiliary inverter in parallel with the test equipment is verified through the B-Box series semi-physical simulation platform.