The dynamic load calculation of offshore wind turbine is an important technology in the future development of wind turbine industry in Taiwan. This paper presents a simplified equivalent-monopile model of reducing the dynamic load simulation time for offshore wind turbines with jacket support structures under seismic conditions. The FAST v7 code that only simulates monopile substructure is chosen as the analysis tool and is further recompiled to include the effect of seismic-pile-soil interaction in the simplified model. The numerical analysis results are then validated by comparing with the results from the fully coupled model, calculated by Bladed software, in a NREL 5MW offshore wind turbine. Two seismic load scenarios, operational and idling conditions, are studied in this work. The results show that usage of simplified model may offer advantages in preliminary support structure design. The advantages are apparent in seismic simulation since the simplified equivalent-monopile modeling allows current FAST v.7 code constraints relaxed for simulating jacket structures and leads to quicker and more convenient dynamic load calculation. The influence of earthquake and soil flexibility is especially important in design considerations since Taiwan is located in a seismically active region. Thus, the developed simplified numerical model will be helpful to shorten the simulation time of a series of seismic design load cases in the preliminary design phase.