Abstract: Mg alloys have been applied more widely as the progressively increased lightweight requirement of the equipment, and it is essential for the dissimilar joining between Mg alloys and steels. However, almost no solid solution and no reaction can be obtained between Mg and Fe, so the interfacial metallurgical bonding cannot be achieved due to the lack of Mg-Fe intermetallic compounds (IMCs). In this study, AZ31 Mg alloy and Q235 steel plates with 3 mm thickness were friction stir welded (FSWed) by adding the AZ61 Mg alloy interlayer to improve the enrichment of the Al element on the Mg-steel interface, and the effect of interfacial element enrichment on microstructure and mechanical properties of FSW joint was investigated. Experimental results show that good welding surface was obtained and Al element enriched at the interface, IMCs formed at the interface from the Al-Fe reaction as increasing the Al content and the bonding strength was improved between the Mg and steel. The tensile strength of FSW joint increased to 192 MPa with an increment rate of 33%, and the elongation increased from 2.8% to 3.1%. This study proposes an effective method to enhance the interfacial bonding and improve mechanical performance of FSW Mg/steel dissimilar joints, which has significant theoretical and engineering values in broadening the application of Mg alloys.