Abstract: With the continuous improvement of lightweight requirements for equipment, Mg alloys are increasingly widely used, and their joining with traditional steel materials is essential. However, Mg and Fe have almost no solid solubility and do not react with each other, so intermetallic compounds (IMCs) cannot be formed to achieve interfacial metallurgical bonding. In this paper, AZ31 Mg alloy and Q235 steel with a thickness of 3 mm were welded by the friction stir welding (FSW) process. The enrichment of Al element at the Mg/steel interface was increased by adding an AZ61 Mg alloy interlayer, and the effect of interfacial Al element enrichment on the microstructure and mechanical properties of the joints was investigated. The results indicate that the weld surface is well formed, and the Al element in the Mg alloy is enriched at the interface. As the Al content increases, Al and Fe react to form IMCs at the interface; the bonding strength between Mg and steel increases; the tensile strength of the FSW joints increases by 33% to 192 MPa, and the elongation after fracture increases from 2.8% to 3.1%. This paper proposes an effective method to promote the interfacial bonding of FSW Mg/steel dissimilar metals and improve the joint properties, which has theoretical and engineering value for broadening the application of Mg alloys.