Abstract: A 20kW CO2 laser was applied to explore the plasma shielding mechanism by studying the effect of plasma height on the welding depth and the focusing laser beam.On the basic phenomenon that the laser induced plasma would sustain for a while even after the laser beam passed through out of the workpiece during high power CO2 laser welding, a simple and effective method is developed to record the effect of plasma on the focusing laser beam.It is found that the laser induced plasma enlarges the diameter of the focused laser beam and the focus shifts downward significantly.This "lensing effect" of the laser induced plasma becomes more serious with the increase of the plasma height.Compared with the absorption of plasma, the "lensing effect" is the main mechanism for plasma shielding in the process of high power CO2 laser welding.