A time-saving, strength-saving, safe and convenient tool special for demounting a hydraulic support pin shaft under coal mines comprises a hydraulic cylinder provided with a liquid inlet and a liquid return port, wherein a hydraulic cylinder plunger is connected into the hydraulic cylinder. The tool is characterized in that a sleeve is fixedly connected at the front end of the hydraulic cylinder, the hydraulic cylinder plunger is located in the sleeve, a connection block is connected at the front end of the hydraulic cylinder plunger, a bolt-penetrating hole is arranged on the connection block, and a connection bolt is arranged in the bolt-penetrating hole. The connection bolt comprises a head portion and a screw, and the internal diameter of the bolt-penetrating hole is larger than the external diameter of the screw portion of the connection bolt and smaller than the external diameter of the head portion of the connection bolt. The connection block is of a U-shaped structure and comprises a connection plate arranged in the middle and lateral plates arranged on two sides, and through holes are arranged on the lateral plates. A pin-penetrating hole is arranged at the front end of the hydraulic cylinder plunger, the connection block is connected with the hydraulic cylinder plunger through a pin, the pin penetrates through the pin-penetrating hole and the through holes arranged on the connection block, and the front end of the hydraulic cylinder plunger is located between the two lateral plates of the connection block.