The invention discloses a device for dismounting a rusted fastened screw nut. The device comprises a base, a fixing bracket, a screw nut wrench, a cap combining wrench, a jack device, an adjusting bolt and a jackscrew, wherein two fixing blocks are arranged at the top of the fixing bracket; the jack device and the adjusting bolt are respectively positioned on two sides of the fixing bracket; an adjusting bolt fixing block is arranged on one side facing to the adjusting bolt, of the fixing bracket; a second threaded through hole is formed in the adjusting bolt fixing block; the adjusting bolt is in threaded connection with the second threaded through hole; the opening of the screw nut wrench and the opening of the cap combing wrench are both positioned between the two fixing blocks; the tail part of the screw nut wrench contacts the lifting output end of the jack device; the tail of the cap combing wrench contacts the head of the adjusting bolt. By adopting the device, a rusted fastenedscrew nut is dismounted by using a jack smartly, the defect that the rusted fastened screw nut is violently dismounted by using a wrench at present is avoided, safe and efficient production can be ensured, both the labor and the force can be saved, and in addition, screw rods and screw nuts can be relatively well maintained.