<p>The copper interlayer effectively suppresses the direct formation of brittle intermetallic compounds (IMCs) at the Mg/Al interface. However, the influence mechanism of extrusion temperature on the interfacial evolution of Mg/Al-Cu composite sheets remains inadequately understood. In this study, Cu-interlayered Mg/Al composite sheets were fabricated via extrusion-shear processing. The effects of extrusion temperature (420, 450, and 480&#xa0;°C) on interfacial microstructure and shear properties were systematically investigated using scanning electron microscopy (SEM) characterization and shear testing. The results show that at 450&#xa0;°C, the interface forms a continuous and dense metallurgical bonding layer (approximately 9.9&#xa0;μm), consisting of Al₃Mg₂ intermetallic compound adjacent to the aluminum side and Al₁₂Mg₁₇ intermetallic compound adjacent to the magnesium side, yielding a maximum shear strength of 30.45&#xa0;MPa. In contrast, at 420&#xa0;°C, the shear strength decreases to 10.75&#xa0;MPa due to insufficient interfacial bonding and void formation, while at 480&#xa0;°C, it further reduces to 13.96&#xa0;MPa as a result of localized molten phase defects and agglomeration of brittle intermetallic compounds. Further analysis of the shear fracture surfaces reveals distinct failure mechanisms: interfacial delamination along the Cu-Mg interface dominates at low temperatures, whereas excessive thickening of the intermetallic compound (IMC) layer triggers brittle fracture at elevated temperatures. This study reveals the evolution of the interfacial structure under temperature modulation and provides a theoretical basis for optimizing the interfacial design and process parameters of high-performance Mg/Al composites.</p>