Abstract: The design and use of magnesium/galvanized steel composite structural components can effectively reduce the weight of the structures and body, and is one of the hot researches for the development of lightweight automotive.In order to reveal the role of Zn layer in magnesium/galvanized steel MIG welding process, this paper compared the weld formation, interface microstructures, microhardness, and corrosion resistance of MIG welded joints obtained from untreated, nitric acid corrosion, and mechanical polishing galvanized steel surface conditions. The study investigated the influence of galvanized steel surface state on the weldability of magnesium/galvanized steel MIG welding. The research results indicated that the presence of Zn layer is beneficial for improving the wetting and spreading properties of liquid magnesium alloy on the steel surface, facilitating the overflow of oxide inclusions in the interface area, promoting interface reaction and the formation of interface transition layer. Moreover, some Zn elements can enter the weld seam area to participate in weld seam alloying. Therefore, the joint weld seam formed by untreated galvanized steel is beautiful, the interface transition layer is uniform and continuous, without welding defects, and the hardness of the weld seam area increases, and the interface area has the best corrosion resistance.On this basis, a model of the action of Zn layer in magnesium/galvanized steel MIG welding was established, which can provide theoretical support for revealing the welding mechanism of magnesium/galvanized steel, improving the weldability of magnesium/galvanized steel, and improving the welding quality of magnesium/galvanized steel. It has important research significance and application value.