The invention provides a bending formation device for thin-walled pipe fittings with complex pipe diameters. The bending formation device comprises a base, a pipe bending module and a supporting module, wherein the supporting module comprises a to-be-bent pipe supporting unit and a bent pipe supporting unit; the to-be-bent pipe supporting unit and the bent pipe supporting unit are fixed to the base and are positioned at a pipe outlet of the pipe bending module; the bent pipe supporting unit comprises a support frame fixed to the base, a plurality of movable pipe socket joints which are continuously arranged, wherein the two ends of each of the movable pipe socket joints are provided with hinged ends, and the hinged ends of the adjacent movable pipe socket joints are movably hinged throughconnecting plates to form flexible pipe sockets adapting to shapes of elbow pipes, and a locking mechanism which is used for locking rotation of the hinged ends of each of the movable pipe socket joints and the connecting plates. According to the bending formation device for the thin-walled pipe fittings with the complex pipe diameters disclosed by the invention, the pipe fittings are divided intomultiple small sections so that bending formation is carried out, and thus, acting force on the pipe fittings can be reduced when the pipe fittings are bent; and moreover, in a process of bending each small section of the pipe fittings, the pipe sockets of the elbow pipes and the flexible pipe sockets can provide uniform acting force to the pipe fittings, so that the stress situation of bending parts is improved.