The key issues in design of deep-sea large-megawatt floating offshore wind power structures are focused on. The OC3 Spar platform of the National Renewable Energy Laboratory (NREL) is applied as the benchmark model of the floating wind power supporting structure. The 5 MW, 6.7 MW and 8 MW wind turbines are selected respectively, and the corresponding floating supporting structure design scheme is proposed using the scale factor design method. Considering the combined action of wind load and wave load, the hydrodynamic analysis and strength analysis of the floating supporting structure under operating conditions and extreme conditions are carried out, and the Response Amplitude Operator (RAO) of the supporting structure and the stress response of the critical nodes of the structure changing with the power of the wind turbine are obtained. The stress reduction scheme of large-megawatt floating wind power structure is proposed. After verification, the structural stress is significantly reduced after improvement.