A method of controlling a plurality of output voltages in a multi-output power supply device for generating a plurality of output powers by using a transformer. The method includes, if a system to which power is supplied from the multi-output power supply device is in a standby mode, blocking any one of at least two output powers of a secondary side of the transformer, wherein the blocked power is supplied to the system; and compensating for a resistance of a feedback circuit connected to a switching controller of a power switch of a primary side of the transformer in relation to one or more unblocked output powers of the transformer and maintaining the output of the unblocked output power to be constant.