A gain control method is provided. In one embodiment, an amplifier amplifies a first signal according to a gain to obtain a second signal, and a converter converts the second signal to a digital signal. First, the gain is sequentially set to a plurality of gain values. A plurality of entropy values of the digital signal corresponding to the gain values is then calculated. A picked entropy value of the entropy values is then determined. The gain value corresponding to the picked entropy value is then determined as a picked gain value. Finally, the gain is set to the picked gain value for amplifying the first signal.