Offshore wind turbines (OWTs) might be subjected to seismic loads with different peak accelerations during operation in the actively seismic regions. The earthquakes might be a potential risk for the OWTs due to its characteristic of stochastic. Earthquake with wind and wave could act on OWT at the same time, thus, the structural responses of such OWTs should be analyzed in consideration of reasonable load combinations. Based on the hydro-elastic similarity, an integrated model of the combined NREL 5MW wind turbine and a practical pentapod substructure is designed. The governing equations of motion of the integrated OWT are established. The dynamic tests and numerical analysis of the OWT model are performed under combinations of seismic, wind and sea load conditions. The El Centro and API-based synthesized seismic waves with different peak ground accelerations (PGAs) are considered in this study. The numerical results are in good agreement with the experimental ones. The coupling effect of the OWT structure under the combined load conditions is demonstrated from the experimental and numerical results. The results indicate that the interaction of earthquake, wind, wave and current should be taken into account in order to obtain proper structural response, especially with small PGA.