The morphology, chemical composition, mechanical properties of the interface reaction products and their effects on the shear strength of the Au-Ge/Cu soldering joint were systematically investigated. The results showed that a close-packed hexagonal (HCP) structure solid solution phase was formed at the interface after soldering at 400 °C for 5-60 min. The composition of the HCP phase varied among 78-46 at.% Cu, 9-42 at.% Au and ~12 at.% Ge. The Young’s modulus and hardness of HCP phase were 105-112 GPa and 4.3-4.7 GPa, respectively. The shear strength of the 5 min soldering joint was 57 MPa; however, it increased to 68 MPa when the soldering time was prolonged to 60 min. The increasing shear strength can be ascribed to the formation of HCP ductile phase and its effect on depleting the brittle (Ge) phase. It suggested that a HCP structured solid solution formed at the interface could enhance the shear strength of the solder joint.