Abstract: The cooling process of the spent fuel assembly stuck in the transfer channel under horizontal natural circulation is studied by FLUENT. The effects of the stuck position of fuel assembly,the length of transfer channel,and the mechanical structure of the fuel transfer system on the cooling effectiveness are numerically studied. The comparison of the simulation results reveals the key factors with effects on the cooling effect of horizontal natural circulation,and provides the necessary basis for the thermal hydraulics analysis for nuclear safety research. Based on the analysis results for the thermal-hydraulics process,an optimized mechanical design for the fuel transfer system is suggested,and the numerical calculation indicates that it can effectively improve the cooling effect,and enhances the reliability and redundancy of the fuel assembly.