To solve the problem of degrading of mechanical properties of as welded 6061-T6 aluminum alloy joint caused by softening of the heat-affected zone, this study applied a double-roller system to perform simultaneous double-sided rolling on 6061-T6 thin aluminum alloy DC double pulse metal inert gas (DP-MIG) welded joints, based on the idea of utilizing welding residual height and large deformation to strengthen the over-aging softened zone. The microstructure and mechanical properties of the welding-rolling composite forming joints were analyzed. The results are as follows: the weld grain is subjected to large plastic deformation by the vertical rolling force; The internal pores disappear, thus the weld area widens. The heat-affected zone adjacent to the weld is subjected to co-macro deformation by the extrusion force from the weld, and the over-aging softened zone far from the weld is subjected to compressive stress from the weld, resulting in partial co-deformation strengthening along the vertical weld direction. The overall hardness of the joints is significantly improved with the hardness of the over-aging softened zone, which increased from 60 − 70 HV to 80 − 90 HV. The tensile properties of the over-aging softened zone are improved. When the welding speed is 800 mm/min, the best tensile strength can reach 83.6% of that of the base metal, 19.4% higher than that of the welded joint, which is a notable improvement to the over-aging softened zone.