This study investigates ultrasonic welding process used to connect two pure copper cables, a pure copper cable with a copper tin-electroplated cable and two tinned copper cables. For the first time, welding mechanism and quality control of tin-plated copper cable joints are systematically studied. It is observed that during welding, the tin from plated side melts and diffuses into copper matrix, resulting in a small amount of tin remaining in areas of joint beyond the weld zone. The presence of tin-plated layer significantly influences mechanical properties of welded joints. The maximum peeling force of joint made with pure copper cables reaches 270.286 N, while the joint made with tin-plated copper cables is only 97 N. Additionally, the failure mode transitions from ductile fracture to a ductile–brittle mixed fracture is investigated. Thermal shock tests were performed on each group of joints to simulate effects of extreme temperature changes in practical applications. After the thermal shock test, the oxidation degree of ultrasonic welded joint increased and the surface of weld became indistinct and dim. However, the tin plating effectively prevents direct contact between copper wire core and atmosphere, thereby reducing the oxidation process. As a result, orange-red oxide forms on the pure copper cable connector, whereas the area in contact with tin in tinned copper cable connector remains unaffected. The welded joint exhibits relatively low resistance in one direction.