Interfacial brittle compounds strongly affect the mechanical properties of Al/Cu brazing joints. It is important to determine the formation, growth behavior and bulk properties of interfacial compounds to further control the interfacial microstructures and properties of joints. In this paper, synchrotron radiation X-ray imaging was used to dynamically characterize the interfacial reactions at Al/Cu brazing joints, and first-principles calculations were subsequently performed to calculate the bulk properties of the interfacial compounds. The formation sequence, modulus and bonding characteristics of the compounds were investigated. The results show that layered Al4Cu9 and faceted and dendritic Al2Cu compounds form during cooling. The faceted and dendritic Al2Cu compound is the primary phase and precipitates at the original interface, exhibiting the characteristics of asymmetric secondary dendrite arms. After the Al2Cu compound contacts the base metal, the Al2Cu compound reacts with the base metal to form the Al4Cu9 compound. The interfacial compounds exhibit mixed bonding characteristics, including metallic and covalent bonding. Compared to those of the Al2Cu compound, the Al4Cu9 compound has a greater cohesive energy, bulk modulus, shear modulus, Young's modulus and hardness, enhancing the hardness of the joints and decreasing their toughness and plasticity.