Abstract: In order to explore the influence of bifurcated tunnel which has special T-intersecting structure on the critical velocity of tunnel fire, FDS is used to construct the scale tunnel models of three structures: bifurcation tunnel with tapered section, bifurcation tunnel without tapered section and straight tunnel. The influence of tunnel shrinking structure and bifurcation angle on fire critical velocity is analyzed by numerical simulation.Research shows that, for the bifurcation tunnel with tapered section, when the local tunnel width at the location of the fire source decreases, the required critical velocity decreases. When the location of the fire source is determined, the tapered section, bifurcation angle and bifurcation structure of the tunnel have no obvious influence on the critical velocity. And a calculation formula of critical air volume at any fire source position in the shrinking section of tunnel engineering is proposed. For the fire at the main road, the relationship between dimensionless critical velocity and dimensionless heat release rate is proposed, which is similar to the variation law of previous linear tunnels, while the higher tunnel height leads to a larger turning point of critical wind speed.

Key words: FDS numerical simulation, bifurcation confluence tunnel, tunnel fire, critical velocity