<p>Pores and weld softening are common defects in laser welding of aluminum alloy, which seriously deteriorate the mechanical properties of welded joints. To solve these problems simultaneously, 304 stainless steel foil is introduced into the weld seam of the laser-welded Al–Mg-Si-Cu alloy joints in this work. It is found that adding 0.01 mm 304 stainless steel foil to the weld seam reduces the porosity of the weld by altering the fluid dynamics characteristics of the molten pool. Meanwhile, presetting 0.01 mm 304 stainless steel foil in the weld seam significantly enhances the tensile strength of the welded joint by 31.9%, while maintaining a good elongation. The prefabricated foils promote the formation of Al₃FeSi in the weld center, improve the distribution uniformity of Si and Cu elements, and refine the equiaxed grains in the weld center. Due to the effects of solid solution strengthening and precipitation strengthening, the strength of the weld is significantly enhanced after adding 0.01 mm 304 stainless steel foil. The prefabricated foil also leads to an increase in weld hardness and the formation of river-like patterns on the fracture surface, indicating a mixed fracture mode of toughness and brittleness. This study proposes a novel approach to regulate the chemistry and thermophysical properties of the weld seam by incorporating 304 stainless steel foil, with the aim of reducing porosity and refining the microstructure, thereby enhancing the mechanical properties of the weld seam.</p> Graphical Abstract <p></p>