Abstract: An optical fiber laser was used to fabricate TC4 titanium alloy welding by direct underwater laser beam welding. The purpose of increasing the penetration depth and protecting the weld were realized by presetting a weld auxiliary to the surface of TC4. The microstructure and mechanical property was analyzed, and the results showed that both the penetration depth and the welding threshold increases, while the cracks reduces. The weld center is mainly composed of primary α and martensitic α' phases, and there is still a large β grain boundary at the bottom of the molten pool. Due to the quench effect of water, the weld underwater has quenched structure, and its microhardness is much higher than that of TC4 base metal. The tensile test specimens of underwater welding are all fractured at the weld joint, and the average tensile strength of the welded joint is 439 MPa, showing brittle fracture.