Abstract: Laser arc hybrid welding is based on traditional laser wire-filled welding by adding an arc to form a hybrid heat source. The change of the heat source mode causes the accumulation of heat generated in the arc zone, which makes the weld grain coarse and reduces the strength and ductility of the welded joint, and ultimately leads to a significant reduction in the service safety performance of the welded structure. The establishment of a three-dimensional molten pool temperature model and mechanical performance analysis to clarify the law of the effect of laser and arc synergistic heat on the strength and ductility of welded joints. The weakening mechanism of the welded joint strength and ductility from the perspective of material microcrystallography are revealed combined with the research on the grain size of the weld, the distribution of grain boundary misorientation, and the texture strength. The results show that the introduction of the arc promotes the heat accumulation of the hybrid heat source and reduces the temperature gradient of the molten pool. The heat accumulation strengthens the preferred orientation growth and texture strength of the crystal grains, resulting in anisotropy of the joint, which reduces the elongation of the welded joint by 2%. The lower temperature gradient will extend the cooling time of the molten pool, promote the growth of grains and reduce the high-angle grain boundaries. Due to the coarsening of grains and the reduction of grain boundary misorientation, it is not conducive to hinder dislocation slip. As a result, the average yield strength of welded joints is reduced by 35 MPa, and the ultimate tensile strength is reduced by 66 MPa. With the increase of the welding current, the high temperature residence time of the molten pool is prolonged, the grain and texture strength of the joint increases, and the joint strength and elongation continue to decrease.