Abstract: Based on the Yangtze River Tunnel on Chongqing-Qianjiang High-Speed Railway, a study is conducted on the distribution characteristics of high-temperature gases caused by the hydration heat of mass concrete in the shield assembly cavern under multi-tunnel condition, as well as the effects of ventilation and cooling. A 3D fluidthermal coupled model is established based on field testing and numerical simulation methods to study the influence of hydration heat temperature, air supply position, and air supply speed on the transient temperature and wind speed variation in the tunnel during various excavation stages. The results show that in complex space conditions, the aggregation of concrete hydration heat leads to higher air temperatures inside the tunnel. During the upper excavation stage, the airflow is relatively simple and flows from bottom to top. In the lower excavation stage, under low wind speeds, the airflow mainly moves upwards, while under high wind speeds, the airflow spreads out in all directions.Ventilation can effectively reduce the air temperature inside the tunnel, but its effect is limited at specific wind speeds. The cooling amplitude increases proportionally with the wind speed, but the cooling rate decreases inversely with the wind speed. During upper excavation stage, the air temperature drop increases with the air supply position's depth; under 40 °C, 50 °C, and 70 °C, the cooling efficiency can be improved by 9.3% to 14.6%. During the lower excavation stage, setting the air supply outlet at the intersection of the assembly cavern and the main tunnel is more conducive to heat removal and can increase the cooling efficiency by 2.2% to 15.5%. When the hydration heat is 40 °C, the air temperature can be reduced to 28 °C through mechanical ventilation and optimized air supply outlet positions. The required wind speed and air supply outlet position for the upper excavation are 1.8 m/s (Position B),and for the lower excavation, they are 4.4 m/s (Position C) and 9.8 m/s (Position D).