The high-speed camera was used to observe the behavior of the melt flow in the high-speed laser welding process. The humping formation and the effect of the welding speed on the humping tendency have been discussed, and the suppression methods of humping have also been  studied. The results show that the humping formation undergoes three stages: nucleation stage, growth up stage and solidification stage. During the nucleation stage, the two side streams flow around the keyhole and subsequently converge at the rear of the keyhole. Finally, the wave motion of the liquid metal, which means the nucleus of the humping, is formed in the converging process. During the growth up stage, the valley of the wave is given priority to solidify, which prevents the peak liquid metal from flowing to the rear of the molten pool. The peak of the wave is filled to expand by the front liquid metal. During the solidification stage, the grown-up peak solidifies from the bottom to top, which lifts the solid-liquid interface upward, declines the contact angle, and makes it difficult for the peak liquid metal to spread. With increasing the welding speed, the distance between the converging position at the rear of the keyhole and the laser beam increases, the temperature of the liquid metal at the rear of the keyhole decreases, the surface tension goes up, the peak liquid metal is hard to spread, the humping is apt to nucleate, and humping tendency is increased. Adopting the laser with small spot diameter is helpful to shorten the distance between convergence position and laser beam, and using the trailing beam of the dual beam laser is propitious to increasethe temperature of the liquid metal in the rear of the keyhole.These two approaches are effective in decreasing the surface tension, and suppressing the nucleation of humping.