An auxiliary power supply device for a multiple-unit train is provided. A normally-open main contact of a three-phase isolation contactor is connected in series between a three-phase four-wire bus of a first unit and a three-phase four-wire bus of a second unit, and a coil of the three-phase isolation contactor is connected in series in a power supply circuit of the first unit and a power supply circuit of the second unit; a control circuit of the first unit controls the power supply circuit of the first unit to be energized when the three-phase four-wire bus of the first unit supplies power, to close the normally-open main contact; and the control circuit of the second unit controls the power supply circuit of the second unit to be energized when the three-phase four-wire bus of the second unit supplies power to close the normally-open main contact.