The invention discloses an oil/air cylinder with a single cylindrical surface guide in the inner cylinder hole and solves the problem that an existing device has a certain concentricity error. The oil/air cylinder is characterized in that a large cylinder opening guide ring is arranged on the part, at the cylinder opening of an outer cylinder barrel, of an inner cylindrical surface, and a medium cylinder opening guide ring is arranged on the part, at the cylinder opening of a medium cylinder barrel, of an inner cylindrical surface; a large cylinder opening locating groove for mounting a large spacing ring is formed in the cylinder opening of the outer cylinder barrel, and a medium cylinder opening locating groove for mounting a small spacing ring is formed in the cylinder opening of the medium cylinder barrel. According to the invention, the inner cylindrical surfaces of the cylinder barrels are used as locating surfaces; the double guiding of pistons and the cylinder openings are realized through the inner cylindrical surfaces of the cylinder barrels; thus manufacture errors of both guide sleeves and mounting surfaces of the guide sleeves are prevented from affecting the concentricity of the guide surfaces; the large spacing ring not only is used for locating the large cylinder opening guide ring but also has a spacing effect on the protruding of the oil/air cylinder, and the small spacing ring not only is used for locating the medium cylinder opening guide ring but also has a spacing effect on the protruding of the oil/air cylinder. The invention especially applies to an oil/air cylinder with a small difference between the oil/air cylinder and the piston rod, as well as a single-action multi-stage cylinder. The oil/air cylinder has the advantages that the running resistance is reduced, abrasion of moving surfaces, sealing elements and the like are reduced, and the processing is simple.