Abstract: Double-sided double-arc MIG welding was applied to weld T-joint of titanium alloy with large thickness for the first time,and the influence of welding parameters on formation and defect forming of T-joint of titanium alloy was systematically studied.The result showed that too large or too small distance between welding gun and workpiece would lead to the increase of weld porosities,which were mainly process porosities.The welding gun angle α₁ should be reduced as far as possible during backing welding,while the welding gun angle α₁ should increase appropriately during filling and cosmetic welding.The incidence of porosity was lowest when the welding gun angle α₂ was about 60°.The larger the heat input of single-arc TIG was,the longer the high-temperature residence time of the molten pool was and the smaller the quantity of porosity was.The ratio of welding current I to welding speed v had an important influence on formation of T-joint.The matching of I and v was improved with the increase of v.Compared with ordinary single-arc TIG,the impact energy A_KV of the cruciform joint prepared by double-sided double-arc MIG welding was significantly improved,and the joint had better plasticity,toughness,and comprehensive performance.