The laser-MIG welding process of medium-thick plate aluminum alloy is simulated by ANSYS, and the distribution of temperature field, residual stress and welding deformation in the laser-MIG composite welding process are obtained. Results show that the temperature field presents an elliptical distribution with the weld as the central axis, and the temperature field is high in the middle and low around the workpiece. Because the equivalent stress of the workpiece is increasing with the welding, during the welding process, a high stress zone of > 300 MPa appears on both sides of the workpiece. With the cooling of the workpiece, the stress concentration of the rigid fixed surface on both sides of the workpiece begins to disappear, and the high stress area disappears after the workpiece is cooled. After the workpiece is completely cooled, the high stress zone is mainly distributed around the welding seam, with the maximum value up to 175 MPa. The stress distribution on both sides of the workpiece is relatively uniform, in the range of 125-130 MPa. After welding, the total deformation of the workpiece presents an elliptical distribution with the weld as the central axis. The maximum deformation on both sides of the weld can reach 0.19, and the weld deformation is 0.09.