Abstract: In this paper, MIG welding was used to weld overlay 304L stainless steel on a T2 pure copper substrate. Based on analyzing the influence of welding current and preheating temperature on the formation of the surfacing layer, the active flux transition layer was further added. By analyzing the effect of the active flux on the surfacing layer and the copper-stainless steel interface, it can be found that the active flux can effectively increase the penetration depth and improve the bonding of the copper-stainless steel interface. It should be noted that Cr₂O₃ can not only produce arc shrinkage but also change the flow pattern of the molten pool, which is better than that of NaF. After the addition of Cr₂O₃ active flux, Cu-Fe in the copper-stainless steel bonding interface forms a miscible zone, a spherical Fe-rich phase on the copper side, and a granular Cu-rich phase on the stainless steel side. The diffusion of Cu-Fe can improve the bonding interface and avoid forming copper-infiltrated cracks. The optimized process is as follows: adding Cr₂O₃ active flux, the preheating temperature at 400 ℃, and welding current 320 A.