The invention discloses a bolt underwater dismantling and assembling tool, and solves the problem that the current bolt dissembling and assembling tool cannot satisfy the requirement of underwater long distance dissembling and assembling operations. The tool comprises a operation rod, an elastic grapping head, the top end of which is connected fixedly with the bottom end of the operation rod. The elastic grapping head comprises an elastic sleeve tube. A locking sleeve tube is positioned on the elastic sleeve tube for closing and opening the elastic sleeve tube. The operation rod comprises an outer sleeve tube, a mobile rod placed in the outer sleeve tube. One end of the elastic sleeve tube is provided with two slide through groove placed in the radial direction of the elastic sleeve tube. The other end of the elastic sleeve tube is provided with a pressing through groove for conducting the deformed tightening and pressing in the pressing process of the locking sleeve tube. The tool can complete the bolt dissembling and assembling operations reliably in the underwater condition with radiation, wherein the clearance between the bolt and the periphery is very small, and the thickness of the bolt head section is very small.