Abstract: Mg alloy and Al alloy were contact-reaction brazed using an intermediate layer of Cu. The joint morphology and interfacial structure were characterized by SEM and EDS, and the diffusion behavior of Cu element was analyzed. It was found that Cu interlayer could effectively block the Mg and Al contact reaction during the joining, thus avoiding the formation of Mg-Al intermetallic compounds. At the same temperature, the diffusion coefficient of Cu element in Mg alloy is much higher than that in Al alloy, which made the Al/Cu interface cannot achieve effective connection below 560 ℃ and Mg alloy many losses occur when temperature is higher than 570 ℃. The applicable range of process parameters was narrow. Mg/Cu/Al contact reaction could be achieved effetely heating at 565 ℃ for 20 min, but the shear strength of joint was only 12.6 MPa. Sound Mg/Cu/Al joint could be obtained under the condition of long-time heating at low temperature firstly and short-time heating at high temperature later. The joint, which was obtained at 475 ℃ for 60 min and then 560 ℃ for 7 min, had a relatively high shear strength of 31.2 MPa.