Abstract: Through FDS numerical simulation, the influence of ambient pressure on the smoke spread law, smoke stratification characteristics, and vertical shaft smoke exhaust effect of tunnel fires was studied. Research has shown that the smoke on the tunnel ceiling in low?pressure environments has a higher temperature, and the longitudinal attenuation curves of the ceiling temperature of the fire source?vertical shaft and downstream area of the vertical shaft follow an exponential distribution. The temperature attenuation coefficient decreases with the increase of ambient pressure; The smoke movement in the fire source vertical shaft area is dominated by thermal buoyancy, and the trend of layering curve change is not obvious. The layering curve in the downstream area of the vertical shaft is greatly affected by ambient pressure. As the ambient pressure decreases, the layering curve shows a clockwise rotation trend, the layering intensity decreases, and the stability of the smoke layer decreases; The smoke exhaust effect of a vertical shaft in a low?pressure environment is better than that of a vertical shaft under normal pressure, and the critical height for the occurrence of suction penetration at the bottom of the vertical shaft has also increased. Therefore, compared with general altitude areas, when a tunnel fire occurs in higher altitude areas, the smoke exhaust effect of the higher vertical shaft is better, the critical height of suction is higher, and the impact on the stability of the downstream smoke layer is smaller, which is more conducive to evacuation and smoke exhaust.

Key words: tunnel fire, smoke exhaust by vertical shaft, ambient pressure, smoke stratification, numerical simulation