The seismic excitations are potential hazards for offshore wind turbines located in earthquake prone zone. To investigate the dynamic characteristics of bottom-fixed offshore wind turbines (OWTs) under earthquakes, the influence of aerodynamic, hydrodynamic loads and pile foundation flexibilities is non-negligible. Hence, in this study, the fully coupled simulation tool FAST V8 for OWTs under earthquakes is updated based on the devised equations and the rotor-nacelle-assembly-tower-substructure numerical models considering different boundary conditions are established. The natural modes of a monopile substructure are compared to validate the accuracy of the established numerical model in the updated FAST V8. Subsequently, the OWT structural dynamic responses with different foundation boundary conditions under the combination of winds and waves, the combination of winds, waves, and earthquakes are analyzed and discussed, and a short-term damage equivalent load is adopted to assess the effect of pile foundation flexibility on OWTs. Finally, it is pointed out that the different foundation boundary constraint conditions have obvious influence on the OWT dynamic responses, especially for the model with coupled spring boundary condition under the action of earthquakes, the effect of second order frequency is more significant. In addition, the influence of different environmental loading frequencies on the OWT structure can also be observed.