The body portion (see the cross-sectional structure) driven by a shaft rotating cylinder, uniformly distributed in the cylinder block about the shaft center line of the plunger is rotated seven, through the center post spring assembly slide shoe against the first variable ( or swash plate) on. Thus, with the rotation of the cylinder and the piston reciprocates complete oil suction and pressure operation. When this type of variable pump output pressure is less than the set-constant pressure, full displacement pressure oil output, namely quantitative output, the output of oil pressure reaches the set pressure, pump flow is automatically adjusted to ensure constant pressure meet the system requirements. Constant output value of the pump, if necessary, in the pressure range can be set stepless adjustment of the structure of the output pressure of the oil at the same time to pass the control variable piston oil chamber and the inlet and under constant pressure valve when the output pressure is less than tune when given constant pressure, constant pressure acting on the spool of the hydraulic thrust is less than the adjusted set spring force, the constant pressure valve is turned on, the variable pressure oil into the piston chamber, a piston variable pressure in the lowest position, the full displacement pump output pressure oil; when it is set when the pump pressure is constant work, constant pressure acting on the spool of the hydraulic thrust equal to the adjusted set spring force, constant pressure valve into the oil discharge port at the same time in the open state, the upper and lower chambers of the hydraulic piston variable thrust equal balance piston variable in a position, if the hydraulic damping (load) increases, the hydraulic transient increases, constant pressure valve oil discharge port to open a large, oil inlet off small, relatively lower variable upper chamber piston chamber pressure lower variable piston moves upward, the pump flow rate decreases until the pressure drops to the constant pressure it is set, then the new variable in the equilibrium position of the working piston. Conversely, if the hydraulic damping (load) decreases, hydraulic transient decrease, constant pressure valve oil inlet opening large drain port off small, variable piston upper chamber relatively lower chamber oil pressure increases, variable piston moves down, the pump the flow rate increases, the pressure rises until it is set to a constant pressure.