Keyhole effect and its periodical fluctuation are important characteristics of deep penetration laser welding processing. The plume fluctuation over laser keyhole arises from the pressure fluctuation of keyhole gas. It is validated by plume behavior obtained from a series of plume images by a high-speed videography system, and the expanding speed of hot gas emitted from laser keyhole in the workpiece is determined. Then, a mathematical model for the simulation of characteristic weld gas shield phenomena during deep penetration laser welding based on a numerical solution of the equations of species, mass, momentum and energy is presented. Important differences are observed between Ar gas shield and He gas shield. All these experiments are also simulated using the 3D hydrodynamic software Fluent. It is seen that the characteristic size of argon gas shield is larger than that of helium gas shield, and the smaller the nozzle inclination angle, the larger characteristic shield size of the assist gas. The effective protection zone will be increased when the flux rate of the nozzle assistant gas is increased. The expansion speed of the plume over laser keyhole is much smaller than 150 m/s. The maximum mass fraction in the species flow zone is smaller than 0.9.