The ultrasonic welding is applied more and more extensively dus to its advantages such as environmental protection, cleaning and energy saving. By researching the mechanism on ultrasonic seam welding of copper plates and tubes for collecting solar energy, it is put forward that the ultrasonic metal welding process can be divided into two stages, and two factors make functions jointly to join the welded metal specimens. In order to successfully join the welding, three basic conditions must be satisfied, that is, there should be high frequency friction vibration in the contact interface; pressure must be imposed on the plate specimen during friction vibration; the time of friction vibration and pressure imposed should be proper. Furthermore, how to select the hardness of copper plate and tube and ultrasonic seam welding parameters is analyzed by experiments.