Floating raft vibration isolation systems in the ships or submarines have the requirement of low weight and small volume. To reduce the weight of floating raft vibration isolation systems and improve its vibration suppression effect， nonlinear energy sink cell （NES cell） is applied to the structural optimization of the floating raft vibration isolation system. NES cells are placed on all the substructures of the floating raft system. The mechanical model of the floating raft system with four degrees of freedom and the vi? bration damping system with NES cell are established. The modal analysis of the floating raft system is carried out. The approxi? mate analytical expression of steady-state response for nonlinear system is derived by Harmonic balance method （HBM） and veri? fied by Runge-Kutta （RK）. The vibration suppression effect under the different total weight and NES cell number is compared by force transitivity response， and the influence of the NES cell number and total weight of the system for the 1st-order mode is ana? lyzed. The results show that NES cell can effectively improve the vibration suppression efficiency of the floating raft system for all the modes while reducing the total weight of the system and realize the structural optimization of the floating raft system effectively.