Abstract: The diameter of the barrel of heavy refining equipment is large. After the barrel is butt welded, when the butt weld is subjected to radiographic inspection (single-wall transillumination), the overall arrangement of the transillumination cannot be effectively observed, resulting in the division and internal division of the weld of the barrel. The external partitions are inconsistent, and the blackness is difficult to meet the requirements of the standard, resulting in rework. In order to solve this problem, the idea of using a level gauge and a laser rangefinder to find the lowest point of the cylinder is proposed and tested. The results show that the method is accurate for the butt weld zone of the cylinder and is worthy of further promotion.