To　reduce　the　levelized　cost　of　energy　(LCOE)　for　offshore　wind　turbines,　a　novel　wind-wave　energy　converter　(WWEC)　with　a　mass-adjustable　buoy　is　designed.　To　analyze　the　impact　of　buoy　mass　variations　on　the　system,　a　coupled　comprehensive　numerical　model　is　established　to　simulate　the　aerodynamics　of　the　turbine　and　the　hydrodynamics　of　the　platform　and　buoy.　It　is　found　that　the　occurrence　of　the　buoy　out　of　water　significantly　reduces　the　output　power.　Adjusting　the　buoy＇s　mass　with　suitable　strategy　can　prevent　the　impact　of　slamming　loads　and　improve　the　power　output.　The　mass　adjustment　strategy　is　determined　based　on　the　output　power　of　the　wave　energy　converter　under　regular　wave　conditions.　It　is　found　that　the　mass　adjustment　strategy　can　significantly　enhance　the　output　power　of　combined　system.　The　buoy　does　not　move　out　of　the　water　under　the　extreme　conditions,　which　avoids　the　impact　of　slamming　loads　on　system　stability.　Moreover,　mass-adjustable　buoys　can　reduce　the　risk　of　mooring　line　failure　compare　to　a　wind　turbine　without　a　buoy.