The invention provides a mounting tool for a large structure on an offshore platform. A rotating end execution mechanism and a hydraulic execution mechanism are arranged on the outer walls of two corresponding sides of the large structure at the upper part; a rotating mechanism and a hydraulic motor are arranged on the outer wall of the large structure at the lower part on the same side with the hydraulic execution mechanism; an aligning execution mechanism is arranged on the outer wall of the large structure at the lower part on the same side with the rotating end execution mechanism; a piston rod of the hydraulic execution mechanism penetrates into the rotating mechanism, and is fixed and locked by a slipping clamping jaw; a T-shaped piston of the rotating mechanism is connected with a driving shaft of the hydraulic motor; a piston rod of the rotating end execution mechanism penetrates into the inner bottom of an aligning execution mechanism housing from an aligning execution mechanism housing and is propped against the inner bottom of the aligning execution mechanism housing; and a hydraulic rotating control system controls the hydraulic execution mechanism and the hydraulic motor to act to drive the rotating mechanism to rotate the large structure, and a hydraulic locking control system controls the rotating end execution mechanism to act to be aligned to the aligning execution mechanism. Through the adoption of the mounting tool, the large structure is positioned quickly and conveniently, the construction risk is reduced, and the construction period is shortened.