Abstract: To achieve cost effectiveness and light weight in automotive manufacturing, copper terminal/aluminum cables welded structures are widely used in cable assemblies. In this paper, the ultrasonic welding process was successfully conducted on aluminum cables with a nominal cross-sectional area of 50 mm² and copper terminals with a thickness of 2 mm, and the effect of welding amplitude on the interfacial microstructure and mechanical property of the joints was systematically investigated, revealing the formation mechanism of the joints. The results show that with the increase in welding amplitude from 39.0 μm to 54.6 μm, the bonding area between the Cu/Al interface increases gradually, and the thickness of the aluminum oxide layer between the Cu/Al interface decreases gradually. Plastic deformation on the Al side is enhanced, promoting dynamic recovery and dynamic recrystallization, and the total shear texture content increases from 7.0% to 17.4%. The average grain size decreases from 22.56 μm to 9.58 μm. Fibrous elongated grains, serrated grains, and fine equiaxial crystals replace the coarse and irregular grains of the base material, and the degree of recrystallization increases from 10.7% to 48.8%. The tensile shear failure load of the joints increases as the welding amplitude increases. When the welding amplitude increases from 46.8 μm to 54.6 μm, the tensile shear failure load increases from 2 235 N to the maximum value of 2 962 N. The fracture location of the joints is shifted from between the copper terminals/aluminum cables to between the aluminum cables.