Assembly welding roundness maintaining tool for low-temperature liquid tank container barrel

The utility model discloses a low-temperature liquid tank container barrel installing and welding roundness maintaining tool which comprises a rigid supporting seat and a flange ring assembly located on the rigid supporting seat, the flange ring assembly comprises an upper half flange and a lower half flange, and the butt joint positions of the upper half flange and the lower half flange are respectively and vertically provided with a Haff butt joint plate. The Haff butt-joint plate of the upper half flange and the Haff butt-joint plate of the lower half flange are clamped and fixed through reamed bolts and nuts; the upper portion of the rigid supporting seat is provided with a semicircular hole used for positioning the outer circle of the lower half flange, and the outer circle of the lower half flange is positioned in the semicircular hole. A cylinder roundness monitoring ring assembly is arranged on the rigid supporting seat, the cylinder roundness monitoring ring assembly comprises a monitoring ring which