In this paper, The ABAQUS software was used to establish a 5083/AZ31B dissimilar alloys model by using the life and death element method. The distribution of welding temperature field, residual stress field, and welding deformation under different laser powers were studied. The results show that the temperature field has an elliptical distribution during the welding process. With the increase of the laser power, the aspect ratio of the weld increases, the maximum welding temperature increases. The temperature field distribution has no effect. Cooling for 20 s after welding, the temperature drops to 160 ℃, and the heat dissipation on the AZ31B side is faster. The heat dissipation will be accelerated with changing the laser power. When the workpiece is completely cooled, the temperature distribution on both sides of the dissimilar alloy plate is semi-elliptical. The residual stress at 3 000 s after welding presents a distribution state of large and small ends in the middle. The longitudinal stress of the workpiece is the tensile stress of 232 MPa at the weld, and the maximum transverse stress is 225 MPa that is also distributed near the weld. The residual stress at the weld toe changes largest. When the workpiece is completely cooled, the post-weld deformation presents an approximately elliptical distribution near the center of the weld and small at the edge of the workpiece. The highest deformation of the workpiece surface is distributed at the end of the weld. The unit deformation is 0.405 mm. The laser power will increase slightly by 0.02 mm deformation amount.