The behavior of flotsam mixed tsunami impacting on land structures and the mitigation effect of mega-floating structures on the water level and hydrodynamic force caused by the offshore tsunami is investigated by a new type of integrated supercomputing. We numerically found that inertial motion and elastic deformation of mega-floating structures can contribute to reduce the tsunami’s seawater-level and to dissipate a hydrodynamic energy of offshore tsunami.  This study can help optimize the strength of seashore buildings and structures against future tsunami threats, and also can help estimate structural damage that can be caused by large-scale natural disasters like hurricanes, storms and tornados, and help to develop effective mitigation tools and systems.