Abstract: A rotary pump which comprises a cylinder having two walls which cover both ends thereof to form a compressing chamber, each of the walls includes a bearing at its center and holes around the bearing to form an inlet or an outlet of the fluid, a shaft supported by the bearings and driven by a motor, a piston having two connected arc surfaces rotatably mounted within the cylinder, each surface of the piston including a groove, an outlet port and an inlet port, a separating plate mounted in the hole of the piston and dividing the hole into two parts and separating the outlet ports and inlet ports on each part, a plunger which may be moved back and forth in the piston hole to open or close the inlet ports. The present invention can pump the fluid in periodic high pressure or stable middle pressure by controlling the movement of the plunger in piston.