To suppress the formation of brittle magnesium-aluminum intermetallic compounds（IMCs）in the welding joints of magnesium-aluminum dissimilar metals and enhance the joint performance, a friction stir welding (FSW) technique was adopted and copper foil was used as an interlayer metal to weld 7075-T6 aluminum alloy and AZ31B magnesium alloy. A comprehensive suite of analytical techniques, including microstructural characterization, energy-dispersive spectrometer（EDS）, X-ray diffractometer（XRD）, hardness testing, and shear testing were employed to systematically investigate the microstructural evolution and mechanical properties of the magnesium/aluminum dissimilar metal joints with and without the copper interlayer. The experimental results reveal that the incorporation of a copper interlayer in the weld nugget zone establishes a ternary magnesium-aluminum-copper diffusion system. This system effectively suppresses interdiffusion behavior between magnesium and aluminum, which reduces the diffusion layer thickness in the weld nugget zone from 150 μm to 50 μm. Simultaneously, new diffusion layers are formed on both sides of the hook-shaped defects in copper-containing joints that effectively suppresses the formation of magnesium-aluminum IMCs, while promoting the formation of copper-magnesium and copper-aluminum IMCs. Meanwhile, the incorporation of the copper interlayer significantly reduces the area of hook-shaped defects at the joint interface and effectively mitigates stress concentration at the interface. Ultimately, compared to conventional joints, the fracture path of copper-containing joints is changed from the IMCs layer to the weld nugget zone. Average shear strength of the joints increases from 3 407 N to 4 615 N, representing a 35.5% improvement in shear performance. The study identifies the causes for the improvement of the mechanical properties of certain magnesium/aluminum dissimilar metals joints through FSW, providing reference for welding other dissimilar materials.