Abstract: In arc assisted activating TIG welding, the base metal is pre-melted by assisting arc with mixture of argon and oxygen to form an oxide layer, and TIG welding is then carried out, so the weld penetration increases significantly. In this paper, two distribution modes of oxygen on the surface of weld pool was proposed based on experimental measurements, and a more complete model of weld pool in AA-TIG welding was developed. The computed results agreed well with the experimental data and existing theoretical results. Combined with the computed results, Grashof number Gr, magnetic Reynolds number Mm and surface tension Reynolds number Ma were employed to investigate the relative importance of buoyance, Lorentz force and surface tension. Peclet number was use to distinguish the relative intensity of heat convection and heat conduction in the weld pool. The results reveal that the surface tension controlled the flow type of weld pool, and the heat convection determined the heat transfer in weld pool. The relationship between the activating elements and weld penetration during A-TIG welding was clarified.