A down-mixing device is provided. 5.1-channel sounds are down-mixed into 4-channel sounds with high-quality multi-channel surrounding sound effect reappeared. A common-acoustic-pole and zero (CAPZ) model is used to rebuild frequency responses of original head-related transfer functions (HRTF). Direct sounds, reflection sounds and late reverberation in a spatial sound field are gathered. Thus, varieties of sounds become rich and 3-dimensional hearing experiences are enhanced with low cost for a 4-channel earphones system.