The effect of ultrasonic vibration coupled with direct current on the wetting behavior of Sn liquid solder on Cu substrate at 270 °C was studied by the wetting balance technique. The wetting balance curves were measured. Results show that the ultrasonic vibration coupled with direct current method significantly improves the wettability of Sn liquid solder. With increasing the ultrasonic vibration power and direct current, the maximum equilibrium wetting force is increased. According to the microstructure observations of interface during wetting process, the ultrasonic vibration coupled with direct current method can increase the dissolution of Cu substrate in the molten Sn solder and promote the precipitation of the intermetallic compounds at interface. The sound pressure distribution inside the molten Sn solder was simulated by a finite element software. The maximum sound pressure occurs at the front end of the ultrasonic probe. During the ultrasonic vibration coupled with direct current process, the chemical driving force caused by the precipitation of intermetallic compounds at interface and the strong convection inside the molten Sn solder jointly promote the movement of the triple-phase line, thereby improving the wettability of Sn solder.