Abstract: In the construction of extra-long tunnels, traditional forced ventilation becomes ineffective due to exces? sive ventilation distances. It is therefore common to install the air chamber at the base of inclined shafts and deploy relay fans in the main tunnel to ensure adequate airflow. To address the challenge of uneven airflow distribution from different chamber outlets caused by varying air demands at different tunnel faces under complex construction conditions, this study takes the 4# cross passage section of a 42 km-long railway tunnel as a case study. A air chamber ventilation system for tunnel construction was designed using numerical simulation. The results indicate that:(1)The closer the two relay fans are in the same main tunnel, the greater the mutual aerodynamic interference;(2) The lower the air velocity inside the air chamber, the more significantly the variation in fan pressure affects airflow distribution;(3) The smaller the air chamber dimensions in the cross passage, the weaker the influence of fan pressure variation on airflow distribution.Based on the simulation results, optimal total pressure parameters for relay fans at each chamber outlet were determined, ensuring consistent air supply under different construction conditions.