The invention belongs to the technical field of nuclear power examination and repair, and particularly discloses a manhole cover demounting and positioning device of a steam generator. The manhole cover demounting and positioning device comprises a guiding rod which penetrates through a manhole cover and is fixedly connected with a manhole seat, and a threaded sleeve which sleeves the guiding rod, wherein the threaded sleeve is a sleeve with a threaded hole in the center of the sleeve; a step is processed at the outer part of the sleeve; a bearing is arranged on the step; the bearing sleeves the upper end of the threaded sleeve; the upper end of the bearing is propped against the lower surface of the manhole cover; and a section above the step of the threaded sleeve is located in the manhole of the manhole cover, and sleeves the guiding rod. Supported by the inherent structure of the manhole seat, the guiding rod extends into a bolt hole of the manhole cover, and the manhole cover slides downwards along the guiding rod by utilizing the gravity of the manhole cover; the guiding rod is connected with the manhole seat through a thread, and through the rotation of the thread, the downward slide or the ascent of the manhole cover is controllable; and besides, a limited part at the lower end of the threaded sleeve is designed to limit the position to which the manhole cover slides down, so that danger generated during operation is prevented.