Disclosed is a butt joint circular seam reverse side gas shielding device for barrel fusion welding. The butt joint circular seam reverse side gas shielding device for barrel fusion welding is characterized by comprising a welding jig and a gas shielding channel. The welding jig comprises a supporting ring (1), eight adjusting screw rods (2) and eight arc-shaped gas shielding grooves (3), wherein the radian of each arc-shaped gas shielding groove is equal to the inner diameter of a barrel. Air inflation holes (7) are formed in the sides of the gas shielding grooves (3), perforated plates (4) which assist in uniform diffusion of gas are added to the interior of the gas shielding grooves, and sealing grooves (8) are formed in the arc-shaped surfaces, making contact with the inner wall of the barrel, of the two sides of each gas shielding groove (3). The butt joint circular seam reverse side gas shielding device for barrel fusion welding has the advantages that the whole device is flexible in structure, adjustable, and easy and convenient to operate, and adapts to barrel circular seams with different diameters, the effect of gas shielding of a welding seam molten pool and a near weld zone is good, oval deformation of barrel welding can be reduced, the shielding gas is saved, and the time needed by gas replacement in the process of gas ventilation in advance is shortened.