Abstract: Laser welding with filler wire is adopted to realize single pass welding and double-sided forming of 4 mm thick TA5 titanium alloy with I-groove, and welding parameters on weld formation, porosity and mechanical properties are studied.Resultsshow that the welding seam is in good shape, and that the tensile strength can reach 797.5 MPa, 98.2% of that of the base metal. With the increase of laser power, the back penetration increases, the weld porosity first decreases and then increases, and the tensile strength increases. When the defocusing amount increases from negative defocusing to positive defocusing, the undercut defects disappear, the porosity first increases and then decreases, and the tensile strength increases. With the increase of welding speed, the back penetration, porosity and tensile strength decrease slightly. The results of metallographic analysis show that in weld zone, α phase has transformed to β phase completely, The rapid cooling rate after welding results in a large number of needle like α′ phase formed in weld zone during the cooling process. In heat affected zone, α phase has partially transformed to β phase was partially destroyed which is mainly made up of bulk α phase and zigzag α phase. The hardness test shows that the hardness of the weld center is the highest, that the hardness of the base metal is the lowest, meaning the hardness from the heat affected zone to the base metal zone decrease. The hardness at the top of the weld is higher than those at the center and the bottom of the weld.