A device and method are provided for automatically adjusting gain, including a conversion module for converting an audio time-domain signal to an audio frequency-domain signal, an analysis module for analyzing the audio frequency-domain signal in accordance with an equal-loudness level contour of human hearing so as to generate strength weightings and generating a signal strength in accordance with the weightings, a calculation module for calculating a gain by analysis of the audio frequency-domain signal when the signal strength falls outside a default range, and a control module for generating an audio output signal in accordance with the gain and the audio time-domain signal.