The utility model provides a DC auxiliary power supply system. The DC auxiliary power supply system comprises a high-voltage DC bus, a medium-voltage DC bus and a low-voltage DC bus which are arrangedon a railway vehicle, a DC auxiliary converter connected between the high-voltage DC bus and the medium-voltage DC bus, and a DC charger connected between the medium-voltage DC bus and the low-voltage DC bus. The high-voltage direct-current bus is connected to an overhead line system or a third rail, supplies power to a traction motor of the rail vehicle and supplies power to the medium-voltage direct-current bus through the direct-current auxiliary converter; wherein the medium-voltage direct-current bus supplies power to each medium-voltage load on the railway vehicle and supplies power tothe low-voltage direct-current bus through the direct-current charger, and the medium-voltage load comprises a variable-frequency air conditioner; and the low-voltage direct-current bus supplies powerto a 110V-grade storage battery and a load of the railway vehicle. According to the utility model, the links of electric energy conversion can be effectively reduced, the electric energy loss is reduced, the operation efficiency of the auxiliary system is improved, the operation cost of the whole auxiliary system is reduced, and the energy-saving effect is improved.