Abstract: In order to study the process of droplet transferring and arc behaviors of AC CMT welding,high speed photography was carried out to capture high-definition images of the arc and droplet of AC CMT welding while the electrical parameters waveforms were gathered by welding electric parameters acquisition system,and the images and waveforms were investigated simultaneously. The results show that the arc of AC CMT goes through the process of arcing-forming droplet-arc blowout. During the electrode positive period,arc concentrates at the end of filler wire and spreads on the surface of base metal,like a bell-jar. During the electrode negative period,the arc crawls up along the filler wire in a way to accelerate the melting of filler wire. Droplet transferring goes through the process of cold-hotcold and no necking forms. During the electrode negative period the melding speed is larger,the melt transfer cycles is shorter and the diameter of droplet is larger compared with the electrode positive period.