Fixed offshore wind turbines usually have large underwater supporting structures.The fluid influences the dynamic characteristics of the structure system.The dynamic model of a 5-MW tripod offshore wind turbine considering the pile–soil system and fluid structure interaction(FSI) is established,and the structural modes in air and in water are obtained by use of ANSYS.By comparing low-order natural frequencies and mode shapes,the influence of sea water on the free vibration characteristics of offshore wind turbine is analyzed.On basis of the above work,seismic responses under excitation by El-Centro waves are calculated by the time-history analysis method.The results reveal that the dynamic responses such as the lateral displacement of the foundation and the section bending moment of the tubular piles increase substantially under the influence of the added-mass and hydrodynamic pressure of sea water.The method and conclusions presented in this paper can provide a theoretical reference for structure design and analysis of offshore wind turbines fixed in deep seawater. Fixed offshore wind turbines generally have large underwater supporting structures. The fluid influences the dynamic characteristics of the structure model. The dynamic model of a 5-MW tripod offshore wind turbine considering the pile-soil system and fluid structure interaction (FSI) is established, and the structural modes in air and in water are obtained by use of ANSYS.By comparing low-order natural frequencies and mode shapes, the influence of sea water on the free vibration characteristics of offshore wind turbine analysis. On the basis of the above work , seismic responses under excitation by El-Centro waves are calculated by the time-history analysis method. The results reveal that the dynamic responses such as the lateral displacement of the foundation and the section bending moment of the tubular piles increase substantially under the influence of the added-mass and hydrodynamic pressure of sea water. The method and conclusions presented in this paper can provide a theoretical reference for structure design and analysis of offshore wind turbines fixed in deep seawater.