The　formation　of　brittle　intermetallic　compounds　(IMCs)　poses　a　challenge　to　Al/Mg　dissimilar　alloys　FSW.　The　results　of　the　implemented　systematic　Cu　interlayer　experiments　show　that　the　diffusion　forms　of　elements　in　the　weld　nugget　zone　(WNZ)　and　banded　structure　zone　(BSZ)　varied　at　different　rotation　speeds　and　Cu　interlayer　thicknesses.　Adding　Cu　interlayer　led　to　the　evolution　of　conventional　aluminium-magnesium　IMCs　towards　aluminium-copper,　magnesium-copper,　and　aluminium-copper-magnesium　IMCs.　The　tensile　properties　of　the　joints　reached　the　maximum　at　lower　rotation　speeds　and　thinner　interlayer　thicknesses,　respectively,　which　was　attributable　to　the　generation　of　Al2Cu　and　Al2CuMg　with　the　best　compressive　and　shear　resistance,　respectively.　Simultaneously,　the　dispersion　distribution　of　Cu　elements　and　phase　transition　weakened　the　stress　gradient　of　BSZ.