Ti–6Al–2Nb–2Zr–0.4B alloy plates were welded using tungsten inert gas welding with a new matching welding wire, and the chemical composition, microstructure, and mechanical properties of the defect-free welded joint were investigated. Furthermore, the influence mechanism of the TiB phase on the mechanical properties of the welded joint was analyzed. The results indicated that the B content in the weld metal is only 0.0145% with trace TiB phase. A large amount of TiB phases precipitate in the fusion zone and aggregate at the grain boundary. The TiB phases in the heat-affected zone are distributed diffusely. The microhardness of the weld metal (295 HV) is the lowest, followed by the heat-affected zone (331 HV), and the fusion zone (341 HV) is the highest. The ultimate tensile strength and elongation of the welded joints are 782 MPa and 12.3%, which are about 93.3% and 77.8% that of the base metal, respectively. The impact toughness of the fusion zone is 22 J, which is 53.7% that of the base metal (41 J) and lower than that of the weld metal (56 J) and heat-affected zone (36 J).