Abstract: The twin-electrode tungsten inert-gas(T-TIG) welding was studied by using the base metal of Q235 low carbon steel plate with the thickness of 3 mm and 4 mm.The results indicate that the T-TIG welding can avoid the defects of undercut and humping effectively in high current and high speed region on the basis of low arc pressure.Accordingly the deposition rate is increased, and the weld formation is improved.In addition, the mechanism of the T-TIG arc pressure reduction to make the sound weld formation was analyzed.