Taking the DTU 10 MW braceless semi-submersible floating offshore wind turbine (FOWT) as the research object, an ideal tuned mass damper (TMD) installed in platform of the 10 MW semi-submersible FOWT is investigated to dynamically compensate the vibrations and reduce the structural loads. Considering the hydrodynamic and mooring effects, a simplified model of FOWT with four degrees of freedom (DOFs) including platform surge, pitch, tower fore-aft bending and TMD translation is established according to the D’Alembert’s principle. Then, based on the Levenberg-Marquardt (LM) algorithm, the unknown parameters related to the simplified model are estimated. Compared with results from FAST-SC, the simplified dynamic model is validated. Furthermore, the response surface method and genetic algorithm (GA) are used to determine the optimized TMD parameter. Finally, the fully coupled time-domain simulation of the FOWT with active TMD subjected to environmental loadings is conducted by using FAST-SC, and the effect of the TMD on the load reduction of the 10 MW braceless semi-submersible FOWT is analyzed. The study provides a feasible method for the structural control of the ultra-large semi-submersible FOWTs.