As a new generation of thermal management materials, diamond/metal composites have shown broad application prospects in aerospace, electronic packaging and other fields. The interface bonding state of diamond/metal plays an important role in the excellent thermal and physical properties of composite materials. Diamond and metal are not wettable and have large acoustic mismatch. Interface modification is an effective way to improve the thermal conductivity of diamond/metal composites. The theoretical calculation and experimental testing methods of thermal resistance of diamond/metal interface are reviewed. The design of diamond/metal interface based on thermal resistance is briefly concluded. Then, the latest research progress of diamond/metal interface modification and improvement of thermal conductivity of composites are summarized from the perspectives of enhancing interfacial strength and introducing interfacial transition layer. Finally, the existing problems in diamond/metal composites research are discussed and the possible development trend in the future is put forward.