During the design phase of a nuclear power plant (NPP), the average temperature of air in the containment was used as an input parameter for the analysis of the Loss of Coolant Accident (LOCA) and Main Steam Line Break (MSLB). Considering the complicated air temperature field distribution in the containment, it is uncertain that if the measurement results of these thermometers in the big space above the main operation platform (MOP) of the Reactor Building (RB) are representative in normal operation. Here we show a method of analysis: based on the Computational Fluid Dynamics (CFD) software, the Three-Dimensional model of the space above the MOP in the RB was established using Computer Aided Design (CAD) software Solidworks, according to the structures, heat dissipation of components, and ventilation in it. The model was meshed using the ANSYS ICEM meshing technology. The numerical simulations and finite element analysis were finally finished using ANSYS CFX software. Then the characteristics of air temperature field and velocity field were obtained. Our results demonstrate the feasibility of the air temperature measurement scheme in the space of this analog, and provide theoretical basis for the correction of the average measured value by comparing the simulation results of the temperature field.