The utility model relates to a device for opening a valve of a fire-extinguishing system, which belongs to the field of fire protection. The utility model is composed of a valve body, a pressing arm, a positioning lever, a rotating arm, a cylinder and a piston, wherein the pressing arm which is pressed on a valve core assembly is installed on the valve body by a pin shaft and is provided with a cylinder piston, and the other end of the pressing arm is connected with the rotating arm by a pin shaft; one end of the positioning lever is installed on the valve body by a pin shaft, the other end of the positioning lever is installed on the rotating arm by a pin shaft, and a supporting arm on the rotating arm is in contact with the cylinder piston on the pressing arm. The utility model has the advantages of simple and reasonable structural design, small volume, easy processing and fabrication, low cost and quick action in opening and closing the valve, and is the device with reliable movement for opening a valve of a fire-extinguishing system.