<p>Offshore structures installed in earthquake-prone areas are threatened by the combined effect of seismic waves and environmental loadings, which should be considered for practical designing the wind turbine system. Despite the widespread investigation of monopile–wheel composite foundations due to their advantages, current research on the dynamic stability and performance of composite foundations under excitation loads of marine environment is limited. To fill the gap, a unique numerical model of offshore wind turbine (OWT) equipped with the monopile–wheel composite foundation subjected to combined loading effects is represented in this study, including the interactions of OWT with surrounding soil and water. The effects of several parameters, such as loading type, seismic intensity, wind speed and wave height, on the dynamic behavior of the whole offshore structure are investigated by the comprehensive studies, capturing efficiently the fundamental mechanisms and general deformation patterns. The numerical results reveal that phase cancellation between wind–wave and seismic loads can lead to amplification of structural responses during specific time intervals relative to the non-seismic case. Moreover, both seismic intensity, wind speed and wave height contribute to the progressive amplification of the time-varying seismic response of the OWT structure, and the influence of seismic intensity on internal forces at the foundation bottom is more pronounced than on the kinematic responses at the tower top. The main findings based on the numerical study are presented for the seismic design codes of monopile–wheel composite foundations.</p>