The utility model relates to a hydraulic expansion type milling cutter, which mainly solves the problem that the existing workover tool can not deal with a bent, deformed and damaged well section. The utility model is characterized in that a valve rod (5) is arranged in a throttle pipe (7) in the middle of a lower partition plate (6) in a main body (1); the lower end of the throttle pipe (7) is positioned in a hollow piston rod (8); a gap is arranged between the external diameter of the valve rod (5) and the internal diameter of the throttle pipe (7); the lower part of the piston rod (8) is connected with a sheath (10) with an annular lug boss; an outer sheath of the piston rod (8) is provided with a spring which is clamped on the upper end of a stair (9); cutter bodies (2) are clamped and pressed in four rectangular holes which are evenly divided on the lower part of the main body (1) through pressing blocks (12); the pressing blocks (12) are fixed on the main body (1) through screws; grooves (13) which are corresponding to the lug boss (10) are arranged in the cutter bodies (2). The hydraulic expansion type milling cutter not only can expand the diameter of a sleeve pipe of a destroyed casing but also can repair the bent and deformed well section of the sleeve pipe of the destroyed casing.