An audio playing system has a first channel output device, a first equalizer, a second channel output device, a second equalizer, a loudness adjusting button, and a controller. The first equalizer adjusts a first channel audio signal with a set of first parameters of frequency response and output it to the first channel output device. The second equalizer adjusts a second channel audio signal with a set of second parameters of frequency response and output it to the second channel output device. The loudness adjusting button generates a loudness instruction. The controller determines an upper limit of loudness based on the set of first parameters of frequency response and the set of second parameters of frequency response, and adjusts a loudness related to the first channel audio signal and a loudness related to the second channel audio signal based on the upper limit of loudness and the loudness instruction.