The invention discloses an underwater wellhead down-setting tool, and belongs to the technical field of offshore drilling tools. The underwater wellhead down-setting tool is designed to solve the problems of complex structures and cumbersome installation and the like in the prior art. The underwater wellhead down-setting tool comprises a core shaft, a driving cylinder, an elastic locking ring and an anti-rotating structure, the driving cylinder and the elastic locking ring are arranged on the core shaft in a sleeving mode, the driving cylinder is in threaded connection with the core shaft, and the anti-rotating structure is used for preventing the driving cylinder from rotating around the core shaft; when the core shaft is rotated, the driving cylinder can linearly move up and down, and the elastic locking ring can be squeezed or loosened by the driving cylinder during the linear up-down movement of the driving cylinder, so that the elastic locking ring can be locked tightly in a underwater wellhead when being squeezed and can be detached from the interior of the underwater wellhead when being loosened. The underwater wellhead down-setting tool is simple and compact in structure, easy to install and operate and more reliable in performance, few parts are used, and after the underwater wellhead is set down to the right place, the underwater wellhead down-setting tool can be detached from the underwater wellhead simply by driving the core shaft to rotate.