In the process of high-powered CO2 laser beam welding, the plasma produced above the welding pool shields the workpiece from the laser beam. In this paper the study about the effect of an added magnetic field on the penetration is made, it is proved that a suitable added magnetic filed can reduce the shielding effect of the plasma to the laser beam. The energy absorption ratio of the workpiece to the energy of laser beam is calculated. And the principle governing the penetration of sudden changes has been analyzed.