Several studies have emphasized the importance of modeling the foundation flexibility of fixed-base offshore wind turbines (OWT). Including the soil–structure interaction (SSI) would give a softer model which reduces the natural frequencies of the turbine and makes them closer to the frequencies of the external loads which may lead to resonance. For the dynamic analysis of OWT, a simplified way is often adopted which simplifies or ignores the rotor nacelle assembly (RNA), SSI, ocean environmental loads, and operational conditions. Considering these, an integrated finite element model (FEM) ABAQUS is developed for monopile OWT including RNA, tower, monopile, and surrounding soil. The reference turbine used in this paper is the National Renewable Energy Laboratory (NREL) 5-MW wind turbine under stochastic ocean environmental loads. The dynamic response by ABAQUS model is compared to the response by the aero-hydro-servo-elastic simulation tool FAST. Default modeling of the foundation in FAST is a rigid connection between the substructure and the seabed, meaning that soil stiffness and damping are disregarded. Two different foundation modeling methods in FAST are investigated in this paper to determine the wind load response of wind turbines, the simplified apparent fixity method, and the improved apparent fixity method. Both methods represent the stiffness of the foundation system by adding one or two fictive beams below the mudline respectively. Finally, sensitivity analysis of wind wave correlation, wind wave misalignment angle, and operation and parked condition are conducted. It was concluded that ignoring foundation fixability leads to underpredictions of the structural dynamic response of offshore wind turbines.