Undercut is one of the most important problems which restrict welding speed. The research indicated that surface flow direction of welding pool near the periphery is the most important reason of undercut occurred. When the existence of surface active element (e.g. the content of S is about 0.02-0.04% in steel) change the sign of surface tension temperature coefficient to positive, undercut always occurs in TIG moveless welding, and not occurs when the content of S in iron is very low( e.g. S% is less 0.01%). Temperature gradient is the most important factor which influences the depth of undercut. The depth of undercut is bigger with the increase of temperature gradient when the other parameters are the same. When filling metal into welding pool, it restrained the molten metal movement from periphery to the pool center. So it can reduce the tendency of undercut, and eliminate undercut as increasing of filling metal ultimately.