The invention discloses a pressure vessel cylinder inner diameter and ellipticity detecting tooling and a using method thereof, which belongs to the field of pressure vessel manufacturing cylinder detection, and overcomes the defects of the prior art that the cylinder of each specification needs to make a set of tooling. The structure of the pressure vessel cylinder inner diameter and ellipticitydetecting tooling has support plates (1), connecting plates (5), a reinforcing plate (6), a supporting beam one (7), supporting circular plates (8), a supporting beam two (9), and the like; the supporting circular plates (8) are provided with two, each of the supporting circular plates (8) is provided with eight rectangular holes uniformly distributed in the radial direction, a pair of connectingplates (5) are installed in each hole, and the supporting plates (1) are welded at the ends the connecting plates; the supporting plates (1) are provided with eight, and the supporting plates (1) areevenly distributed outside the circumference of the supporting circular plates (8); the supporting beam one (7) connects two supporting circular plates; and the supporting beam two (9) is evenly distributed at the radial position of the supporting circular plate (8) to reinforce the connection of the two supporting circular plates (8). The using method of the pressure vessel cylinder inner diameter and ellipticity detecting tooling is that: the connecting plates (5) are adjusted so that the top end distance of the two symmetrical supporting plates (1) is less than the inner diameter of 3 to 5mm required by the pressure vessel drawing, and the device moves in the pressure vessel shell for multi-point detection. The pressure vessel cylinder inner diameter and ellipticity detecting tooling has certain universality and is convenient to assemble and disassemble.