A standby power supply device includes at least one port, at least one current detection unit, a power supply unit, and a control unit. Each port is configured to connect to an external powered device. A current detection unit is connected to one port. Each current detection unit detects the current of the port. When the current of one port exceeds a first predetermined value, the current detection unit outputs a first detection signal. The power supply unit supplies standby power to an external powered device. The control unit controls the power supply unit supply power to the external powered device when the control unit receives the detection signal from the corresponding current detection unit. When the control unit receives multiple first detection signals, the control unit controls the power supply unit to supply power to multiple external powered devices simultaneously.