In present work, two combined wind turbine (WT) and wave energy converter (WEC) systems have been concerned. One is a fixed-bottom system, referred as “MWWC” (monopile-WT-WEC combination); the other is a floating system, referred as “TWWC” (TLP-WT-WEC-combination). Comparative study of the hydrodynamic responses of the two combined systems has been done by numerical simulations in the time domain. Hydrodynamic loads of the supporting structures and the WEC are calculated by the AQWA code, which is available for modeling multi-body systems including both mechanical and hydrodynamic couplings between the supporting tower and the WEC floater. Firstly, the effect of different power-take-off (PTO) parameters, wave periods and the displacements of the WEC on the performance of the WEC’s wave energy production of the two combined systems under typical wave cases has been investigate, and preliminary optimal values for the PTO damping stiffness of the two combined systems have been obtained and compared; secondly, the effect of the horizontal contact force between the supporting tower and the additional WEC floater of the two combined systems have been further investigated, which is important for both the fatigue and extreme loads design of the supporting tower. Finally, a new strategy for MWWC system by adding horizontal PTO dampers between the supporting tower and the WEC floater has been proposed and investigated, which is helpful for both reducing the horizontal contact force and using the relative horizontal motion to produce power.