Abstract: The structure-borne acoustic emission (AE) signals during pulse MIG welding of aluminum alloy were detected in real-time and analyzed to investigate the time and frequency characteristics of metal droplet transfer. The results show that the order and periodicity of metal droplet transfer during pulse MIG welding were characterized by AE signals waveform and AE event of metal droplet transfer. As the pulse frequency and welding heat input increased, the metal droplet transfer was transformed from short circuiting transfer mode to spray transfer mode. The volume of metal droplet was refined gradually, which showed decreasing characteristics of AE energy release. With short circuiting transfer mode, the frequency domain of AE signals of metal droplet transfer was broad and distributed around the high frequency region. With spray transfer mode, the frequency domain of AE signals of metal droplet transfer was narrow and distributed around the low frequency region. As the metal droplet transfer was transformed from short circuiting transfer mode to spray transfer mode, the energy variation induced by metal droplet showed a parabolic trend.