Abstract: In the study,the tuned mass damper（TMD）is applied to a monopile offshore wind turbine to reduce the structural dynamic responses under winds and waves. The apparently fixed（AF）model is used to simulate the flexibilities of pile foundation. Then,the boundary conditions of the offshore wind turbine（OWT）and TMD coupled analysis model in FAST-SC are updated,according to the theories of AF model. Meanwhile,in order to perform the optimization design of TMD,the simplified coupled model of OWT and TMD is established based on Lagrange equation. The fist bending mode of the support system of OWT is reserved in the simplified model,and the TMD is simplified as a single degree of freedom system. Sequentially,the optimized TMD parameters are derived using the response surface method. The vibration control effects of TMD are studied based on the coupled responses of OWT under wind and wave loads,and the influence of pile foundation flexibilities on the mitigation effects are proved. It can be seen that the different optimized TMDs are derived owing to the differences of OWT dynamic characteristics caused by pile foundation flexibilities. Further,the more effective reductions of OWT with pile foundation flexibilities using the optimized TMDs are observed.