Abstract: In this study, the application of pure argon and argon-hydrogen mixed gas ( Ar + 5 % H 2 ) in K-TIG welding of S30403 stainless steel was discussed, and the effects of hydrogen on weld formation, joint microstructure and mechanical properties were analyzed. The arc morphology under two protective gases was observed by high-speed camera technology, and the influence mechanism of hydrogen on welding process was systematically studied by electron backscatter diffraction ( EBSD ) and microhardness test. The results show that the arc under the protection of argon-hydrogen mixed gas is more concentrated, the arc diameter is significantly reduced, the arc stiffness and heat source energy density are improved, and the weld penetration is increased. In addition, the weld microstructure under argon-hydrogen mixed gas protection showed more uniform and refined multi-orientation characteristics, and the maximum texture index decreased from 33.96 under pure argon protection to 21.52. The addition of hydrogen significantly reduced the ferrite content in the weld ( from 19.7 % to 11.7 % ) and improved the plastic deformation ability of the weld. Although the tensile strength decreased slightly ( from 664.4 ± 2.3 MPa to 658.2 ± 1.6 MPa ), the elongation increased ( from 50.46 ± 0.89 % to 53.23 ± 0.68 % ). The study also found that the penetration under the protection of argon-hydrogen mixed gas is the weld under the protection of pure argon, and there are no defects such as pores in the weld, which further verifies the optimization effect of hydrogen addition on the welding process.