A piston-cylinder structure includes a cylinder, an end cap, a tapered portion for extruding dead space, an upper bowl-shaped seal portion, a columnar seal portion, a lower bowl-shaped seal portion and a piston push rod. The upper bowl-shaped seal portion is configured to form sealing between the piston push rod and the cylinder when the piston push rod moves toward the end cap and to remove crystals on an inner wall of the cylinder. The columnar seal portion is configured to enable the piston push rod to be coaxial with the cylinder, and to provide a mechanical support for the piston push rod. The lower bowl-shaped seal portion is configured to provide sealing between the piston push rod and the cylinder and remove the crystals on the inner wall of the cylinder when the piston push rod moves toward the end cap.