Abstract: Laser welding seam of titanium alloy is a non-ideal acicular structure. The research on its superplastic deformation mechanism can promote the application of titanium alloy LBW/SPF technology, and also have benefits of significance to the development of material forming mechanism. The results show that the laser welding of TC4 titanium alloy after welding have good superplastic deformation ability. During the deformation process, two important microstructure changes are taken placed in the weld, that is, the microstructure transformation of the acicular structure lamellae and the lamellar tissue equiaxed. Under the action of stress, the lamellar structure is transformed into equiaxed grains through the process of fracture, disintegration and equiaxed. The dynamic recrystallization and compression is the main mechanism of the lamellar fracture; the mechanism of the lamellar disintegration is grain boundary sliding and rotation, the plastic flow mechanism of the weld joint is grain boundary sliding and rolling mechanism.