Abstract: Triple-wire double-pulse MIG welding system has been built up in this test. The welding process was monitored by the high-speed photography and electrical signal collection system. The droplet transfer modes of triple-wire double-pulse MIG welding of aluminum alloy and the influence of double-pulse frequency on weld bead formation were studied. The results of experiment show when weak pulse transferred to strong pulse, the droplet transfer mode was one drop per pulse. When strong pulse transferred to weak pulse, the re-ignition of wire occurred during the beginning of transition and the droplet transfer mode was one drop two pulses during the end of transition. With the increase of the double-pulse frequency, fish-scale pattern of weld bead was more and more dense. When the double-pulse frequency was 3 Hz, the density of fish-scale pattern was uniform and weld bead formation was the best.