Abstract: The aluminum-magnesium composite structure can take advantage of excellent properties of the both,and has huge application potential in workpieces such as armor shells and aircraft fuselages.Friction stir welding （FSW） can effectively avoid defects such as porosity and slag inclusion,and has great advantages for dissimilar welding.However,in the process of friction stir welding of dissimilar aluminum-magnesium alloys,the welded joint is prone to produce hard and brittle aluminum-magnesium intermetallic compounds,which seriously deteriorates the mechanical properties of the welded joint.The influence of process parameters on the weld forming of 3 mm thick 5A06 aluminum alloy and AZ31B magnesium alloy FSW joint was studied,and the range of welding process parameters with good weld seam were obtained.By observing the cross-section of the weld,divide it into 5 areas,namely the heat affected zone at the Mg side,the thermal mechanical heat affected zone at the Mg side,the stirring zone,the heat affected zone at the Al side,and the heat affected zone at the Al side.And using SEM and XRD analysis,it is found that the intermetallic compounds of aluminum-magnesium dissimilar alloy FSW are mainly Al₃Mg₂ and Al₁₂Mg₁₇.Most of the fracture occurs at the Mg side.At the junction of thermal mechanical heat affected zone and the stirring zone at the Mg side,lots of Al₁₂Mg₁₇phase is produced,which is the main cause of fracture.