Abstract: Although Be/2024Al composites possess high strength potential, they often exhibit brittle fracture due to Cu-rich Al–Cu phases and Al–Cu–Mg phases. To address this critical issue, this work proposes a customized heat treatment process designed to modulate the distribution of interfacial Cu. This process successfully promotes the diffusion of Cu atoms from the Al matrix across the interface into the Be phase, where they exist uniformly as a solid solution. This effectively mitigates the localized enrichment of Cu at the Be/Al interface and eliminates brittle Al–Cu interfacial phases. First-principles calculations theoretically confirm that the solid solution of Cu at the Be/Al interface is beneficial for enhancing interfacial bonding strength, increasing the work of adhesion (W_ad) of Al/BeO and Be/BeO interface by 39.2% and 113.3%, respectively. Benefiting from this, the fracture mode of the composite undergoes a fundamental transformation. Compared to the pre-heat-treatment state, the optimized samples show a slight increase in strength, but a remarkable improvement in ductility, with increases of 403% and 311% in the O-temper and T6-temper states, respectively. This achieves a successful brittle-to-ductile transition in the Be/2024Al composite, offering a promising approach to resolve the poor ductility and machinability of large-scale, as-cast Be/Al components. Furthermore, this study reveals the mechanism of cross-interface Cu diffusion and elucidates the influence of the interfacial BeO layer on this process.