A power management method is disclosed. The power management method comprises the step of a computer system checking existence of a manufacturing identifier (ID) of a human interface device (HID) when the computer system is operated in a first mode; the computer system continuously detecting whether the HID exist when the manufacturing ID exists; the computer system starting a timer when the computer system detects that the HID does not exist; the computer system entering a second mode when the timer expires; the HID determining whether the computer system is operated in the second mode, when plugged into the computer system; the HID performing state transition on the computer system when determining that the computer system is in the second mode; and the computer system entering the first mode when detecting the state transition.