In this paper, a numerical model is proposed to investigate the keyhole dynamic behaviors and the formation process of hump. Both of the welding processes with and without shielding gas are calculated during the simulation. It is revealed that the hump generated in the weld is caused by the metal flows in the rear part of the keyhole aperture driven by the resultant force of surface tension, recoil pressure and buoyance force. The shielding gas can significantly change the fluid flows in the molten pool and hence suppress the formation of the hump. The proposed method is validated by the confirmation experiments and good agreement between the simulated and experimental results have been found.