High-speed metal-active gas (MAG) arc welding experiments were conducted to determine the critical welding speeds at which humping defects occur, the spacing between two adjacent humps along the welding direction, and the cross-section morphologies of the hump and the valley in a humping bead. Based on the weld pool images sensed during high-speed MAG welding process, the forming mechanism of humping bead is analyzed and verified through up-hill and down-hill welding experiments. Meanwhile, the effect of the compositions of shielding gas on the weld bead formation in high--speed MAG welding is also analyzed.