A circuit and a method for controlling the power mode of a class-D amplifier are provided. The power mode control circuit comprises an amplitude detector and a duration detector. The amplitude detector provides an amplitude signal according to the absolute signal level of an input signal. The duration detector provides a shut-down signal to a switching control circuit of a class-D amplifier according to the length of the time period in which the amplitude signal is at a predetermined state, wherein the switching control circuit is turned on or turned off in response to the shut-down signal.