Abstract: Self-fusion welding for I-groove of small diameter stainless steel pipes with wall thickness greater than 3 mm is prone to incomplete penetration defects, which seriously affects their welding quality. Automatic self-fusion tungsten inert gas (TIG) welding method was adopted for I-groove of 304 austenitic stainless steel pipes with outer diameter of ϕ32.5 mm and wall thickness of 3.25 mm, and influence of welding current and initial arc preheating time of welding parameters on formation of weld was studied through numerical simulation and welding tests. The corresponding relationships between welding current, initial arc preheating time and forming parameters of weld were obtained by numerical simulation, and the preliminary ranges of welding current and initial arc preheating time for self-fusion welding tests of pipes were determined. Then, the specific welding current and initial arc preheating time for good formation of weld were obtained through automatic TIG welding tests. The test results showed that when welding speed was 100 mm/min and welding current was 105 A or 115 A, the middle section of circumferential weld of pipes could achieve penetration, and the weld would not produce excessive penetration. When welding current was 115 A and initial arc preheating time was 4 s or 5 s, weld width at the back of initial welding section was relatively stable, which could ensure reliable penetration of weld.