A time-domain method is developed for modeling the dynamics of a floating truss-structure wind turbine with multiple rotors mounted on the deck of the platform. In its hydrodynamic aspect, a hybrid panel-stick model is built up incorporating the potential flow theory to evaluate the wave inertia force and a Morison strip method to evaluate the wave drag force. The proposed analysis model is validated against a 1/50 scale test of a semi-submersible floating wind turbine, which was carried out in Kyushu University. Good agreement between the simulation results and the experimental data confirms the validity of the developed method. Further numerical simulations are performed in a set of wind and wave conditions to investigate the effect of wave drag force on the dynamics of the floating wind turbine. The results show that applying a hybrid panel-stick model is fairly effective to reduce the unphysical large resonant responses.