Abstract: The convection difference between normal speed and high speed GMAW processes was investigated using the numerical simulation, a high speed photography system was used to capture the transient images of the weld pool, and the humping formation in high speed GMAW process was also discussed. The results show that at the longitudinal sectional view of the weld pool behind the arc, both counterclockwise circulation and outward fluid flow pattern exist in normal speed GMAW process, only counterclockwise circulation exists in high speed GMAW process. Two main factors are responsible for the humping formation, one is that the high momentum of the backward fluid flow causes the initiation and growth of swelling, and the other is that the unbalance of normal surface tension force in the welding direction promotes the shrinkage of liquid channel. Any measures that help reduce the surface tension can inhibit the formation of humping.