Thermodynamic properties in a fluidized bed nuclear reactor were simulated by means of two-fluid model coupled heat conduction model. Neutron transport equation was used based on group diffusion method. The effect of velocity of inlet coolant and different packing height on the temperature distribution was studied. Results showed that the gas temperature was uniform in a fluidized bed nuclear reactor. The temperature fluctuation strengthened with the velocity of inlet coolant in the annular and center zones, and the gas temperature was raised with the increase of packing height. The simulation result of instantaneous gas temperature was in an agreement with the calculation results of Pain et al.