Abstract: On the bases of the study for the mechanism of the increasing of A-TIG welding penetration, a new method of activating TIG welding, the laser aided activating TIG welding was proposed.At first, the surface of weld was melt by the mini power laser protected by oxygen.As a result, the oxygen content increases in the weld pool surface.Then the conventional TIG welding was used to cover the weld.The oxygen could change the direction of fluid flow in molten pool, which leads to the weld beads with narrower width and deeper penetration.Without the activating flux, the penetration can be increased by 2 times.There is no slag in the surface of weld and the appearance is good.The oxygen can change the temperature dependence of surface tension gradient from a negative value to a positive value;and cause the significant changes in the weld penetration.Fluid flow could be inward along the surface of the weld pool toward the center and then down.This fluid flow pattern transfers heat to the weld root and produces a relatively deep and narrow weld efficiently.This change is the main cause to increase the penetration.