Double-wall UL storage tank adopts the process of canning and insertion welding. The canning and canning joints are rotated and closed with the special shrinking equipment and head equipment imported from the United States, and then the canning joints are inserted on the special fitting machine, and then welded with the automatic submerged arc welding machine imported from the United States. The stiffness and strength of the tank are greatly increased by the plastic deformation of the shrinkable mouth and the interference insertion between the shrinkable mouth and the cylinder joint. The strong welding through the double steel plate enables the storage tank to withstand high temperature, without deformation and welding after a long time of fire, and then maintain the dense structure and no leakage.