Abstract: The 3D flow characteristics in SG can provide input for the analysis of flow induced vibration (FIV). The secondary side flow field was simulated based on the porous media model with FLUENT solver. The flow resistances of flow along and cross tubes as well as flow resistances of downcomer, support plates and separators were added to the momentum equation. The 3D heat transfer from primary to secondary side fluid was calculated during iteration and set as the energy source of secondary side fluid, and the calculation results agree well with the design values. Meanwhile, the results show that the resultant localized thermal-hydraulic characteristics were unevenly distributed. The maximum and minimum flow vapor qualities flowing into the primary separators are 0.75 and 0.07, respectively. The average heat transfer coefficients of primary and secondary sides are 15 856.5 and 63 623.0 W/(m²•K), respectively. The maximum heat transfer coefficient of secondary side is 122 862.9 W/(m²•K). The average heat flux of U-tube is 149.9 kW/m². The maximum cross flow velocity and cross flow energy (ρu²) through the U-bend region are 4.06 m/s and 1 145 J/m³, respectively.