Abstract: To achieve high-quality welding of Cu/Al dissimilar metals, friction stir lap welding was performed with copper placed on the upper layer and aluminum on the lower layer, and the microstructure and mechanical properties of the Cu/Al dissimilar metal weld were systematically analyzed. Results indicate that when the rotational speed is 600 r/min, and the welding speed is 30 mm/min, the weld surface is smooth with good forming quality. The copper-aluminum interface has good bonding without obvious defects, and a “hook-like self-locking” structure is formed at the Cu/Al interface, which increases the effective contact area between copper and aluminum and improves the interfacial strength. In addition, mutual diffusion layers and intermetallic compound layers can be observed at the Cu/Al interface. The microhardness of the upper copper layer in the weld zone is significantly higher than that of the lower aluminum alloy layer, and the thermal softening effect of the aluminum alloy is more remarkable during the friction stir welding process. Meanwhile, the hardness of the weld transition zone is higher than that of the base metals, which is mainly caused by the intermetallic compounds. The maximum tensile load of the Cu/Al lap joint is approximately 1.68 kN, and its tensile strength is about 30.5 MPa. Plastic deformation occurs on both the aluminum and copper matrix sides in the weld fracture morphology, indicating that the bonding between copper and aluminum is firm under this process. High-quality joining of Cu/Al dissimilar metals is achieved by adopting the configuration with copper on the upper layer and aluminum alloy on the lower layer.