Abstract: A-TIG welding has obvious function of increasing welding penetration compared with conventional TIG welding.In this paper, welding flow field and temperature field of nimonic 263 with TIG and A-TIG were simulated and the simulated weld shape and penastration were compared with the experimental results.It is found that the depth of welded molten pool of A-TIG is deeper than that of TIG welding and its width is smaller when the welding current is the same.Moreover, with increasing welding current or decreasing travel speed the depth of welded molten pool of A-TIG increases linealy and the increase is more remarkable for the A-TIG than the TIG welding.The mechanism of increasing welding penetration by A-TIG welding is further deduced by simulating the fluid flow mode in the welded molten pool.It is indicated that flow mode in welding pool is the major factor which causees the increase of welding penetration.The flow rate in the center is obviously larger than that at the edge of the welded molten pool and the circular flow from the outside to the inside is the leading flow direction and brings the fluid with high temperature in the edge to the center which leads to the increase of metal melting rate in the center than that in the edge.Besides, the effect of increasing welding penetration is enhanced constantly by increasing welding current.