The ultrasonic energy is transmitted to the welding area through the upper weldment. Due to the large acoustic resistance at the interface of the two welding areas, local high temperature will be generated. Due to the poor thermal conductivity of the plastic, it can not be distributed in time for a while, and gathered in the welding area, resulting in the rapid melting of the contact surfaces of the two plastics, plus a certain amount of pressure, so that they are integrated. When the ultrasonic stop, let the pressure continue, some of the allowable holding time, make it solidify, thus forming a solid molecular chain, to achieve the purpose of welding, the welding strength can be close to the strength of the raw material body.