Abstract: In order to study in detail the air flow characteristics and temperature distribution in the demonstration fast reactor pit, and verify the rationality and cooling effect of the current pit ventilation system, the three-dimensional numerical simulation of the air flow basin of the demonstration fast reactor pit under normal operating conditions was carried out by using CFD software. The results show that the cooling effect of the ventilation system meets the design requirements, and the maximum temperature of the inner wall of the concrete is 50.7 ℃. However, the internal flow field of the pit is complicated and the temperature distribution is uneven, and the ventilation system import and export layout needs to be further optimized. The calculation results provide more accurate boundary conditions for the thermal calculation of the main vessel and the penetration support, and provide a reference for primary circuit design of the demonstration fast reactor.