Abstract: The simplification of geometry, the generation of grid and the method of post processing of 5×5 rod bundles in a typical fuel assembly were discussed in detail. The influence of spring and arches on pressure drop and mixing characteristics was simulated using 5×5 rod bundles with one span model and the interaction between multiple grids was calculated using 11-span 5×5 rod bundles without spring and arches. The simulation results indicate that spring and arches may enhance lateral flow and heat transfer with the increase of Nu 8%. However, the pressure drop caused by spring and arches is 40% larger than that of the simplified model without spring and arches. Flow features and heat transfer characteristics across grids were obtained using the 5×5 multi-span model which can be used as CFD analysis reference of thermal-hydraulic characteristics in the fuel assembly design development and prediction of DNB.