A system for automatically adjusting a volume of a loudspeaker includes a loudspeaker and a processor. An infrared sensor, which generates a sensing signal when sensing IR radiation from a heat-radiating source, is disposed on the loudspeaker. The sensing signal provides a size information commensurate with a size of the heat-radiating source. The processor receives a plurality of the sensing signals at different time points to obtain a plurality of corresponding size information, realizes a change of distance between the heat-radiating source and the loudspeaker according to a change of the size information, and generates a volume-adjusting signal according to the change of distance between the heat-radiating source and the loudspeaker. The volume-adjusting signal is transmitted to the loudspeaker to adjust the volume of the loudspeaker accordingly.