Abstract: Taking 1.5 mm TC4 titanium alloy sheet as an object, three-dimensional transient finite element model of back reflection induced synergistic laser weld was established theoretically and stress field of back reflection induced synergistic X-shaped laser weld was obtained by indirect coupling based on temperature field of the molten pool. The results show that the way of back reflection induced synergistic laser welding can obtain X-shaped weld under the laser processing parameters, while the way of conventional laser welding (without reflection plate on the back of sheet) only obtained V-shaped weld. Meanwhile, although the longitudinal residual stress on the upper surface of the weld was basically identical to conventional laser welding, residual stress on the lower surfaces of the weld was bigger than conventional laser welding and reached the same level as the upper surface. Moreover, the change of longitudinal residual stress of back reflection which induced synergistic X-shaped laser weld was very small with different line energy. It indicates that sensitivity of line energy to residual stress was relatively low.