Floating photovoltaic platform is recognized as one of the most potential technology development direction in the industry. How to maintain the strength and stability of the structure under harsh marine environmental conditions is of great significance for the development and application of far-reaching marine solar energy. Based on practical engineering, a hexagon truss type floating photovoltaic platform is designed in this paper, which uses buoyancy block to provide buoyancy and adopts a 6 × 1 spread mooring system. The whole platform has the advantages of small waterplane, stable platform desirable motion response and small mooring tension. Firstly, an integrated numerical model of truss-buoyancy block and mooring system is established in commercial software OrcaFlex, subsequently the dynamic coupling calculation is carried out in the full-time domain, and the platform’s motion response and mooring tension characteristics under extreme sea conditions with high/low tide levels are investigated, respectively. The results show that the platform has excellent hydrodynamic performance as well as the tension on mooring chains is small, which can maintain stable operation under extreme sea conditions. The analysis and results can provide important reference for practical engineering.