Abstract: Ultrasonic welding was used to connect BVR2.5 Cu cables and BLV6 Al cables. SEM, EDS, XRD and universal tensile testing machine were used to investigate the effect of ultrasonic welding energy on the bonding properties for Cu-Al cables joints. The results show that effective joining of Cu-Al cables could be achieved by ultrasonic welding. With the increase of welding energy, the diffusion distance of the atoms at the Cu-Al interface increases, but the intermediate phase is not formed, and the tensile strength of the joint increases and then decreases. At low welding energies, the tensile strength of the joint depends on the metallurgical bonding area of the Cu-Al conductor. When the welding energy exceeds 300 J, the tensile strength of the joint depends on the effective bearing area of the Al side. The joints obtained peak tensile load (409.8 N ± 8.9 N) at the welding energy of 300 J and fractured on the Al side in tension. The fracture was in ductile fracture, which indicates the good performance of the joint.