Abstract: Narrow gap TIG welding was carried out for 36 mm thick TA5 titanium alloy. After welding, 100% radiographic testing (RT) and 100% penetration testing (PT) were performed for the joints. The microstructure, tensile, hardness and bending tests of the joints were carried out to study the microstructure and properties of the narrow gap TIG welding joint. The results show that the weld is silver white, the forming is beautiful, and the ray and penetration test results of the joint are 100% qualified. The tensile strength of TA5 titanium alloy joint can reach 718 MPa, 95.7% of the strength of the base metal, and there is no visible crack on the inner and outer surfaces after 90° bending. From the weld to the fusion zone to the heat affected zone, the hardness gradually increases, and tends to be stable in the base metal. In addition, there is a softening zone during the transition from the weld zone to the fusion zone, but the hardness decrease is not significant. The weld structure is only serrated α phase, and the structure is coarse columnar crystal. Compared with the microstructure of the base metal (equiaxed α+ intergranular β), due to the influence of the welding thermal cycle, the grains in the heat affected zone near the base metal grow obviously, while the solid phase transformation occurs in the heat affected zone near the weld side, resulting in the irregular arrangement of the serrated α structure. In addition, the morphology of the tensile fracture is ductile fracture.