The invention discloses an extensible displacement pump. The extensible displacement pump includes a pump body and a work cavity, an air cylinder mechanism is arranged in the work cavity and includesa right shell body fixedly connected with the inner wall of the work cavity through a first fixing ring, a lower sliding groove is formed in the wall of the work cavity, a lower sliding block is slidably arranged in the lower sliding groove, a ring-shaped sliding groove is formed in the wall of the right shell body, a ring-shaped sliding block is slidably arranged in the ring-shaped sliding groove, and a left shell body is fixedly connected with the ring-shaped sliding block. According to the extensible positive displacement pump, when the pumping speed needs to be adjusted, power supply poweror the impeller speed does not need to be adjusted, the thickness of a piston can be adjusted and extension of an air cavity is adjusted, the adjusting process is simple and fast, steps required foradjusting are reduced, the adjusting difficulty is reduced, and practicability is improved greatly.