The invention discloses a flange rounding machine and belongs to the technical field of machine manufacturing, aiming to achieve the objectives that elliptical error in radial dimensions of an annular forging stock with the diameter greater than five meters can be corrected to the minimum and production cost can be lowered. The technical scheme includes that the flange rounding machine comprises a rectangular bed and a hydraulic pressure station, a wall pillar A and a wall pillar B are transversely arranged from left to right at the front end and the rear end of the bed, an isolation cavity is arranged between the wall pillar A and the wall pillar B, and a concave rectangular notch is formed from the top down in the middle of the bed. A hydrocylinder is parallelly arranged on the right side of the rectangular notch, a cushion anvil block A and a cushion anvil block B are arranged on the left side of the rectangular notch, and a lower anvil cushion is arranged at the bottom of the rectangular notch. When the flange rounding machine is in use, the annual forging stock is lifted and placed on a balancing support frame through a girder vehicle and lapped on the lower anvil cushion of the rectangular notch to be joined and positioned with the cushion anvil block A and the cushion anvil block B. The flange rounding machine is simple in structure.