Abstract: The aim of this study is to investigate the post-welding deformation of TC4 titanium alloy thin plates under different constraint conditions during butt laser welding, providing references for welding fixture design and deformation control. Using numerical simulations of TC4 titanium alloy plate butt laser welding established in SYSWELD, three constraint conditions were designed to analyze the residual stresses and plate deformation of the welded joints under different constraint conditions, and the deformation states of the simulation results were compared with those of the validation experiments. The results show that, when comparing the simulation results under the three constraint conditions, the test plates under constraint conditions 2 and 3 exhibit residual stresses in both the transverse and longitudinal directions that contract toward the weld seam. Under constraint condition 1, the test plates only exhibit contraction at the arc start and arc end points. The deformation patterns of the test specimens in the validation tests under constraint condition 1 were consistent with the simulation results, indicating that constraint condition 1 effectively improves the post-welding deformation of TC4 flat plate butt laser welding.