The invention discloses a cavity assembly welding and fastening device. The cavity assembly welding and fastening device solves the problem of uncontrollable form and location tolerance caused by a plurality times of cavity welding. The cavity assembly welding and fastening device comprises a base, a top positioning structure, first supporting columns and an adjusting mechanism; the assembly is positioned between a bottom supporting structure and the top positioning structure; the top positioning structure is positioned at the top of the assembly and is used for fixing and adjusting placementposition of the assembly; the base is used for supporting and fixing the assembly; the base is in fixed connection with the top positioning structure by the first supporting columns; and the adjustingmechanism is used for adjusting levelness of the assembly to placing the assembly on a horizontal surface. Vacuum sealing connection of a plurality of crucial positions of a cavity being completed simultaneously is guaranteed, the condition of the uncontrollable form and location tolerance caused by multiple times of machining processes is avoided; and one-off welding of the cavity is achieved and the cavity can be used for a long time, so that yield rate of cavity assembly is greatly improved.