Abstract: The 8 mm thick TA5 titanium alloy is welded by IPG fiber laser, and the microstructure and mechanical properties of the welded joint analyzed. The result shows that the surface of the laser welded joint is continuous, uniform and with no splash, and there are no such internal defects as pores and cracks. The microstructure of base metal is equiaxed α phase, fine and uniform. The microstructure of weld zone is mainly composed of coarse β columnar grains, a large amount of acicular martensite α' and a small amount of lath martensite. The structure of heat affected zone is mainly composed of equiaxed α phase, a small amount of acicular martensite α' and a small amount of residue β. At the boundary of the fusion line, columnar grains and equiaxed grains are crossly crystallized and epitaxially grown, so as to ensure the stable connection of the welded joint. The Vickers hardness of each area of the welded joint varies greatly, the highest hardness exists near the fusion line, and the hardness of weld zone and heat affected zone is significantly higher than that of base metal. The tensile fracture exists at the base metal, away from the weld, indicating the tensile strength of the laser welded joint is equal to or slightly higher than that of the base metal. The high tensile strength of the weld is due to the net-basket structure formed by a large amount of acicular martensite.