Abstract: The T-shaped fillet weld composed of 30 mm thick TC4 panel and 15 mm thick TC4 web was welded by double-sided double-arc MIG method. The droplet transition morphology was observed by high-speed photography, the temperature distribution of single-sided welding and double-sided double-arc welding was compared by numerical simulation, the weld formation after double-sided double-arc welding was observed, the microstructure characteristics of the welded joint were analyzed, the microhardness and the tensile properties of the joint were tested. The results show that the welded joint with good shape and no internal defects can be obtained by double-sided double-arc MIG welding. The welding droplet transition form is a continuous droplet transition, and the transition process is stable and uniform.When welding at both sides, the welding temperature field of both sides is symmetrically distributed. The metallurgical bonding of the joint is good, the weld zone is coarse columnar crystal, and the interior structure of the grains is long needle α', lamellar α and block α.The heat affected zone at the panel side is residual primary α, lamellar α and residual β.The heat affected zone at the web side is needle shaped α, lamellar α and residual β.The microhardness of the heat affected zone at the web and panel sides is higher than that of the base metal.The tensile strength of the joint reaches more than 969 MPa, and the fracture position is located at the base metal.