First-principles calculation and experimental methods are used to study the interfacial properties and reaction of diamond/Al composites. Based on the first-principles method, the interfacial adhesion work (W_ad), electronic structure and charge transfer of diamond/Al models are calculated systematically. The results show that the adhesion work of diamond(100)/Al(111) is 41% higher than that of diamond(111)/Al(111), therefore, the interface bonding of diamond(100)/Al(111) interface is stronger. According to the analysis of the electronic structure, there are more charges transferring at the diamond(100)/Al(111) interface, and the high charge density is distributed on the side of C atoms. The redistribution of charges at the interface is conducive to the formation of Al—C bond, so that the tendency of forming Al—C bonds is greater. The introduction of Al—C bond can promote the formation of C—C bond at the diamond(100)/Al(111) interface and improve the interfacial adhesion work. In addition, the diamond/Al composites are fabricated by vacuum gas pressure infiltration, and multi-scale characterization of the interface structure of diamond/Al composites is carried out. The interfacial debonding occurs mainly on the diamond 111. Meanwhile, the interface product Al₄C₃ is easier to form on the diamond 100. The experimental phenomenon is consistent with the calculated results. Moreover, the influence of the interfacial reaction on the properties and stability of diamond/Al composites are further discussed through heat-moisture treatment. The study finds that the performance degradation in heat-moisture environment is related mainly to the hydrolysis of the interface product Al₄C₃. After 60 days’ heat-moisture, the thermal conductivity of the diamond/Al composites decreases by 29.9%, and the bending strength is reduced by 40.1%. The large attenuation of performance is not conducive to the stability of composites in complex environments. Therefore, inhibiting the formation of Al₄C₃ and improving interfacial selectivity are of great importance in developing the performance and stability of diamond/Al composites. The research in this paper not only lays a theoretical foundation for the first-principles calculation of the interface properties of diamond/metal, but also possesses important guidance significance in designing the diamond/metal composites.