Underground fixed cylinder type single acting pump of coal-bed gas well rodless drainage system

The utility model relates to an underground fixed cylinder type single acting pump of a coal-bed gas well rodless drainage system. The underground fixed cylinder type single acting pump comprises an outer pipe, a power pump cylinder, a power plunger, a plunger connecting pipe, a drainage pump cylinder, a drainage plunger and an upper one-way ball valve, and further comprises an upper outer pipe plug, a plug sealing sleeve, a pump cylinder connecting and limiting connector, a lower outer pipe connector, a lower connector and a lower one-way ball valve. By means of the technical scheme, when a pump set is in a natural vertical state, the drainage pump cylinder in the pump set is fixed to the lower outer pipe connector through threads to assist the pump set in tightening and fixing; as a result, the situation that the power plunger and the drainage plunger are locked due to bending deformation of the pump set which is stressed non-uniformly is eradicated, the service life of the pump set is greatly prolonged, and maintenance cost is lowered; high-pressure liquid is filtered, so that wear is lowered; through arrangement of the pump cylinder connecting and limiting connector, the lower movement limit is set, and the working efficiency and safety of the underground pump set are greatly improved.