A device and a method for adjusting power consumption are provided. The method is adapted for adjusting power consumption of an external graphic processing unit (GPU) according to different voltage sources and includes following steps: determining whether power to the external GPU is supplied by a first voltage source or a second voltage source according to a comparison result of an input voltage of the external GPU and a default reference voltage. If the input voltage is higher than or equal to the default reference voltage, power to the external GPU is supplied by the first voltage source, and the external GPU is controlled to remain in a normal speed operation status. If the input voltage is lower than the default reference voltage, power to the external GPU is supplied by the second voltage source, and the external GPU is controlled to remain in a low speed operation status.