Abstract: For DTU（Technical University of Denmark,DTU）10 MW wind turbines,the fully coupled analysis for the semi-submersible floating offshore wind turbine（FOWT） system was conducted by using FAST software under the typical sea states with different hydrodynamic components to compare and analyze the effect of second-order difference-and sum-frequency wave forces on the dynamic responses of the ultra-large semi-submersible FOWT,including only first-order wave force,first-and second-order difference-frequency wave forces calculated by Newman’s approximation and full-field quadratic transfer functions（QTFs）,respectively,first-and secondorder sum-frequency wave forces calculated by full-field QTFs,and first-and complete second-order wave forces calculated by full-field QTFs. Based on the fatigue calculation theory of the wind turbine structure,the effect of second-order difference-and sum-frequency wave forces on the fatigue failure of the wind turbine structure is analyzed. The study found that under extreme sea states,second-order differencefrequency wave force has a significant effect on the platform motions,and the second-order difference-and sum-frequency wave forces has a significant effect on the structural loads and fatigue failure of the semi-submersible FOWT. In addition,the Newman’s approximation method obviously underestimates the effect of second-order difference-frequency wave forces on the dynamic response of the semisubmersible FOWT.