Abstract: To obtain reasonable scales and grid requirements of computational zone, water flow in 5×5 fuel pins assembly was simulated adopting incompressible N-S equations, standard k-ε turbulent model and SIMPLE algorithm. By comparing pressure drop, velocity and its vertical component at different planes perpendicular to fuel pins, with relative error smaller than 5%, conclusions were obtained. Length of inlet part and outlet part of computational domain should be about 2.0 times and 2.3-2.8 times of space between two spacer. Polyhedral mesh should be adopted and its scale should be smaller than 0.35 mm in zone between spacer as smooth pin zone adopt structured grid with 0.3 mm-0.5 mm dimension.