Abstract: T-joint of 6063-T6 aluminum alloy was prepared by metal inert gas (MIG) welding and then coated. The influence of the post-weld coating process on the structure and properties of the joint was studied. The results show that the coating process had no effect on the grain morphology and size of each region of the joint. The amount of acicular β″ phase precipitation in heat affected zone increased significantly after coating. The mechanical properties of the joint were significantly improved after post-weld coating; the tensile strength was 13% higher than that before coating; the microhardness of the heat-affected zone was 18.8% higher than that before coating. The weakest area of the joint was the heat affected zone before coating, and was the weld zone after coating. Magnesium and silicon atoms were enriched in the heat affected zone to form a solute atoms-rich zone, the GP (Guinier & Preston) zone; meanwhile, the increase of acicular β″ phase precipitation disturbed the movement of dislocations, resulting in the significant increase of the hardness of the heat affected zone, and making the weld seam the weakest area of the joint.