The invention relates to the technical field of forming, assembling and welding of a large-scale supporting cylinder on a lightering platform on the sea, and provides a pre-welding shaping auxiliary mold of the large-scale steel cylinder and a welding method. The shaping mold is composed of a welding platform, a fixing column, a slide way, an outer lead screw, an outer arc locating block, an inner ejector rod and the like. The outer circle of the cylinder is shaped through an adjusted rigid restrained position; shaping accuracy of the cylinder is ensured through the lead screw, a seam width positioning wedge, a process support and the like; a welding process of the cylinder is made; tack welding is firstly conducted on a longitudinal seam, then the longitudinal seam is integrally welded, and the welding sequence is that welding is conducted from the middle towards the two ends at intervals; sectionalized barrel bodies are welded in an assembled mode and through an X-type double-face groove, the butt-jointing groove faces upwards, the inner side of the cylinder is welded firstly when welding is started, carbon arc air gouging back gouging is conducted on a welding seam in the outer side after the cylinder is turned over, and then the outer side of an annular welding seam is welded. Slight welding deformation is achieved through adjustment of a rigidity restraint positioning block and changes of the welding sequence, and requirements for welding accuracy of the cylinder are met.