The hydraulic cylinder is the executive element in the hydraulic transmission system. It is the energy conversion device that converts the hydraulic power into the mechanical energy. The hydraulic motor realizes the continuous rotation movement, while the hydraulic cylinder realizes the reciprocating motion. The structure of hydraulic cylinder has three categories: piston cylinder, plunger cylinder and swing cylinder. Piston cylinder and plunger cylinder realize reciprocating rectilinear motion, output speed and thrust, swing cylinder reciprocating swing, output angular speed (speed) and torque. In addition to the single use of the hydraulic cylinder, it can be combined two or more or combined with other organizations. To accomplish special functions. The hydraulic cylinder is simple in structure and reliable in work. It has been widely used in the hydraulic system of machine tools.