A series of technological parameters in controlling plasma in high power CO 2 laser welding were investigated by using a precise declined blow system for assist gas regulating and controlling. The plasma light intensity above molten pool and in key hole was also measured by phototransisters. The plasma control mechanism was theoretically analysed. The results show that plasma control effect was divided into three regions with the change of assist gas flow quantity. No matter what gas species were used, plasma can be best controlled only was the assist-gas operation pressure slightly higher than the metallic vapour pressure.