The invention discloses a demounting tool for a spent fuel grillwork of a nuclear power plant. The demounting tool comprises a bracket, a long rod tool, a supporting guiding device and a fastener adapter head, wherein a height-adjustable guiding wheel is mounted on the bracket; the long rod tool is removably connected to the guiding wheel of the bracket in a lapped mode and can slide on the guiding wheel; the supporting guiding device is mounted on the long rod tool, the height and the width of the supporting guiding device are matched with the height and the width of a to-be-demounted sleeve,and the supporting guiding device is used for supporting guiding and centering of the long rod tool in the sleeve when a fastener of the sleeve is demounted; and the fastener adapter head is detachably fixed to the front end of the long rod tool and matched with the fastener of the to-be-demounted sleeve. Compared with the prior art, the demounting tool for the spent fuel grillwork of the nuclearpower plant is simple in structure, convenient to operate, high in demounting speed and safe in use.