The application discloses a vacuum welding device, a mounting method and a using method. The left and right sides of the bottom base of the vacuum welding device are provided with side wall adjustingcolumns correspondingly, the bottom base is provided with a table face for storing a U-shaped structure main body, the upper portion of the bottom base is provided with a top pressing late, and the top pressing plate is fixedly connected to the bottom base through a first fixing piece; and the upper part of each side wall adjusting column is located at the front portion of the upper part of the longitudinal part of the U-shaped structure main body and provided with an adjusting screw in the front and back direction, and a magnetic shielding adjusting screw rod is connected with the position, located on the outer side of each side wall adjusting column, of the bottom base. According to the vacuum welding device, simultaneous completing of vacuum sealing connecting of multiple key positionsof the main body is ensured, the uncontrollable geometric tolerance due to multiple processes is avoided, the vacuum welding device is suitable for assembly welding under the high temperature vacuum environment, the geometric tolerance of the key positions is ensured, the fact that the path of a radon atom is smooth and not deflected is ensured, one-time welding and long-term use of the main bodyare achieved, and the yield of a cavity assembly is improved.