The thread is not cut on the crosshead body, but the bolt is fastened with the flange clamp connection. However, when the disassembly and assembly, the bolt installation space is limited, the special tool is inconvenient and difficult to apply force, and the tightening torque is difficult to reach the force of each bolt. Consistently, the clamping flange and the crosshead end face cannot guarantee the uniformity of the four sides. Nowadays, large and medium-sized compressors mostly adopt hydraulic nut connection structure, which consists of hydraulic pump, high-pressure hose, pressure gauge and tensile body. The tensioner is installed at the axial position of the tail end of the piston rod and the end surface of the crosshead, and is axially stretched by a pure pulling force, so that the elastic deformation zone is elongated in the middle of the crosshead end of the piston rod, and the diameter is slightly deformed to achieve a strict rated pressure. The amount of stretching, after the outer nut lock is tightened, the oil pressure is removed to achieve the connection fastening. The utility model has the advantages of simple and convenient operation, small volume, light weight, compact structure, high strength, stable and safe use, high efficiency and quickness.