Abstract: Al/Mg bimetallic materials combine the excellent properties of aluminum and magnesium alloys, offering the promising application prospects in aerospace and other fields. Diffusion bonding is one of the most effective methods for fabricating the bimetallic materials. However, the tendency for brittle Al–Mg intermetallic compounds to form at interface significantly reduces the bonding strength. To address this issue, the Ni/Cu composite interlayers were introduced at the Al/Mg bimetallic interface to achieve the solid/liquid diffusion bonding. The effects of holding time on the interfacial microstructural evolution and bonding strength were systematically investigated. The results show that at the bonding temperature of 500  ℃, the eutectic reaction occurs between the Mg matrix and the Cu foils, promoting the liquid-phase diffusion at the interface, while the solid-state diffusion takes place between the Ni foils and the Al matrix. As the holding time increases from 15  min to 30  min, the Cu foils are gradually consumed and eventually disappeared. When the holding time reaches 45  min, the wavy interface morphology begins to form. The maximum shear strength of 53.78  MPa is achieved after holding for 60  min, the interfacial structure is composed of Al/Al₃Ni₂+Al₃Ni/Ni/Mg₂Ni/Mg₃AlNi₂/Mg, and the fracture occurs near the interface between Mg₂Ni and Mg₃AlNi₂.