The present invention relates to a pump, which comprises a pump body, a pair of check rings and a pneumatic control valve. When the pneumatic control valve is operated, a main shaft in the pump body is enabled to reciprocally move in a shaft hole, thereby enabling two diaphragms to be stretched and compressed at the same time for performing the compressing and the sucking stroke for the liquid; when subject to the hydraulic pressure, an arc-shaped convex ring of each of the check rings is deformed, and a contract ring is contracted for enclosing the main shaft thereby forming a liquid checking effect; when each of the check rings is not subject to the hydraulic pressure or the air pressure, the arc-shaped convex ring is not deformed thereby forming a slit between the contract ring and the main shaft.