A piston device and its manufacturing method are disclosed. The piston device comprises a piston body having at least one annular groove on the side wall of the piston body, and at least one piston ring made of flexible material and mounted in the annular groove. The outer diameter of the piston ring mounted on in the annular groove is larger than the diameter of the piston body, so that the piston ring contacts with the inner wall but the piston body has no contact with the inner wall while the piston device moves reciprocally at the axial direction of the cylinder.