According to the use of overflow valve ,pressure reducing valve and sequential valves. (1) Overflow valve: Can control the hydraulic system to maintain the constant state when the pressure is reached. The overflow valve used for overload protection is called a safety valve. When the system fails and the pressure rises to a limit that may cause damage, the valve opens and overflows to ensure the safety of the system. (2) Pressure Reducing valve: It can control the branch circuit to get the stability of the oil lower than the main circuit. The pressure relief valve according to its control function is different, can be divided into the fixed value pressure reducing valve (output pressure is constant value) ,fixed difference pressure reducing valve (input and output press difference is fixed value) and the fixed ratio relief valve (input and output pressure to maintain a certain proportion). (3) Sequential valve: Can make an executive component (such as hydraulic cylinder, hydraulic motor, etc.) after the action, and then in order to make other components action. The pressure produced by the pump first pushes a hydraulic cylinder movement, at the same time through the order valve inlet function, when the hydraulic cylinder movement is completed, the pressure rises, the effect of the upward thrust is greater than the spring adjustment value, the valve core rise makes the inlet port and the oil outlet interlinked, makes another hydraulic cylinder movement.