The invention belongs to the technical field of thin-wall housing machining, and particularly relates to an anti-deformation device for a major-diameter thin-wall housing welding boss. The anti-deformation device comprises a supporting ring, locating shafts and glands; the diameter of the external cylindrical surface of the supporting ring is the same as the internal diameter of a housing; the supporting ring is used for the location of the lower end faces of T-shaped blocks; holes having the same center lines as holes in which bosses are mounted are formed in the supporting ring; the locatingshafts are used for the location of the supporting ring through the holes in the supporting ring; the glands are arranged at the upper ends of the locating shafts and used for the location of the upper end faces of the T-shaped blocks; and supporting devices are arranged at the other ends of the locating shafts. The problem that the welding deformation quantity is not easy to control when nonuniform bosses are welded at the major-diameter thin-wall housing is solved, the component deformation amount when the nonuniform bosses are welded at the housing is controlled within a tolerance range, and the process specification requirement is met. The anti-deformation device is novel and light in structure, reliable in location, convenient and flexible to operate and convenient for manufacture.