Abstract: The wetting behavior of molten 4043 aluminum alloy on the surface of different steel plates and the relationship of wetting behavior and process parameters during cold metal transfer welding were investigated by using laser backlight technology. The role of zinc coating of galvanized steel in the wetting process was further analyzed. The results show that the coupling relationship of wetting behavior of molten Al 4043 alloy on Q235 steel in CMT process can be expressed as δ(t)=δ₀+2(54-10v)(1-exp(0.33-3v)t (when v≤4 m/min), and δ(t)=δ₀+(58-7v) exp -(1.7-0.15v)t (when v > 4 m/min). Good wettability was observed when the heat input was large whether the used substrate was Q235 steel or galvanized steel. Zn vapor induced the instability of metal transfer process, because the Zn vapor on the interface cut off the contact of molten droplet and base metal.