Abstract: In order to obtain the thermal hydraulic characteristics of the low temperature heating reactor, a CFD calculation model of the low temperature heating reactor based on the annular fuel assembly was established. In the modeling process, because the structure of the annular fuel assembly was very complex, in order to reduce the calculation amount, the porous medium model was selected for simplified modeling. In order to obtain the key parameters used by the porous medium model to characterize the pressure drop, a refined 11×11 annular fuel assembly numerical model was built. In the process of refined modeling, 62 million meshes were used for CFD calculation. The boundary conditions were given according to the actual working conditions. The physical properties of the fluid were given by the inlet temperature and working pressure under the design conditions of the fuel assembly. Several groups of pressure drop data for the annular fuel assembly were obtained. According to these data, the porosity and permeability of the porous medium model used to characterize the pressure drop characteristics of the annular fuel assembly were obtained. Since the porosity and permeability of the porous medium model were obtained, the annular fuel assembly was simplified to a cuboid in the modeling of the heating reactor. It turns out that this approach greatly reduces the amount of mesh. Three numerical models with different mesh numbers were built. The sensitivity analysis of the mesh and turbulence model was carried out using the inlet and outlet pressure drop and the outlet average temperature. It is found that the inlet and outlet pressure drop and the outlet average temperature were not sensitive to the mesh and turbulence model. The standard kε model and 8.24 million meshes were used for subsequent calculations. The fluid was simplified as an incompressible fluid, and the physical parameters were determined according to the inlet parameters of the heating reactor. The distribution of the power factor along the axial direction was not considered in the calculation in this paper, but the distribution of the radial power factor was considered. The thermal hydraulic properties of the low temperature heating reactor were calculated. The results show that the porous medium model can correctly simulate the resistance characteristics in the core, and can better predict the overall distribution trend of the parameters. The temperature field of the core is related to the radial heating power, and each fuel assembly at the core outlet has a certain radial temperature difference. The core inlet flow distribution is relatively uniform, and the maximum flow distribution nonuniformity coefficient is 1.07. The results of this paper have reference value for the design of thermal hydraulic safety with low temperature heating reactor based on annular fuel assembly.