Abstract: An equivalent model methodology for studying the thermal-hydraulics problems of spherical fuel assembly water cooled reactor is proposed.In this paper,3D geometry modeling,meshing and simulation calculation of flow and heat transfer for narrow-gap small bundle with annular fins are performed with computational fluid mechanics software,and the pressure drop and heat transfer coefficient with the velocity ranged from 0.076 to 0.334 m/s are obtained and compared with the existing experimental data for sphere bed.Based on comparison results,geometrical parameter for the equivalent models is optimized to get the most optimized equivalent model.The study result shows that the most optimized model is equivalent to sphere bed on characteristics of flow and heat transfer,and proves the feasibility to study the thermal-hydraulics problems of spherical fuel assembly water cooled reactor by the equivalent model methodology.