The fully coupled analysis model of a jacket offshore wind turbine (OWT) is established based on the governing equation of motion of the structure which is derived in accordance with the blade element momentum theory (BEM), Morison formula and theories of structural dynamics. The dynamic characteristics and structural responses of the jacket OWT under the different combined seismic cases are analyzed. It can be seen that the interactions of the wind and wave loads are non-negligible in the seismic analysis of an OWT, and the abundant dominant frequencies of the responses of the support system under the combined seismic cases are discovered. Meanwhile, the passive control method of tuned mass dampers (TMD) is applied to the support system of the OWT under the earthquakes, and the influence of the TMD parameters on the reduction of the responses of the support system are investigated. Furthermore, according to the reduction of the structural responses, the suggestions for the design of a TMD under the combined seismic cases for the bottom fixed OWT are summarized.