Abstract: Al/Mg bimetallic materials combine the excellent properties of aluminum and magnesium alloys, offering promising application prospects in aerospace and other fields. Diffusion bonding is one of the most effective methods for fabricating such bimetallic materials. However, the tendency for brittle Al-Mg intermetallic compounds to form at the interface significantly reduces the bonding strength. To address this issue, a Ni/Cu composite interlayer was introduced at the Al/Mg interface to achieve solid/liquid diffusion bonding. The effects of holding time on interfacial microstructural evolution and bonding strength were systematically investigated. The results show that at the bonding temperature of 500?°C, the eutectic reaction occurs between the Mg matrix and the Cu foil, promoting liquid-phase diffusion at the interface, while solid-state diffusion takes place between the Ni foil and the Al matrix. As the holding time increased from 15?min to 30?min, the Cu foil was gradually consumed and eventually disappeared. When the holding time reached 45?min, a wavy interface morphology began to form. The maximum shear strength of 53.78?MPa was achieved after holding for 60?min. At this stage, the interfacial structure was composed of Al /Al3Ni2+Al3Ni/Ni /Mg2Ni/Mg3AlNi2/Mg, and fracture occurred near the interface between the Mg?Ni and Mg3AlNi? phase layers.