Abstract: The humping bead formation mechanism in the thick-wire(φ 3.2) high-speed MAG welding process was studied. Using the high-speed camera system,the arc shape and weld pool flow were investigated. By analyzing the captured image, the viewpoint of dynamic equilibrium point was presented. The distance between the dynamic equilibrium point and the arc is the cause of humping bead formation. Any factors to prompt the dynamic equilibrium point to move to the arc,will prevent formation of humping bead. In contrast,any factors to prompt the dynamic equilibrium point away from the arc,will lead to the appearance of humping bead. Furthermore,the uphill and downhill welding experiments were designed to verify the correctness of the above discussion about humping bead formation mechanism.