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Small longitudinal velocity cannot prevent backlayering in tunnel fire, while excessive longitudinal velocity will destroy stratification of smoke layer and lead to bifurcation flow. As smoke bifurcation flow proceeds, the longitudinal flow is divided into two streams and flow along both sidewalls of the tunnel ceiling. The critical velocity of bifurcation flow is the minimum value at which bifurcation flow starts to occur. To investigate the critical velocity of bifurcation flow, experiments and CFD simulations were conducted. Experiment was carried out in a reduced-scale tunnel, which is 8 m long, 1 m wide and 0.5 m high. The numerical research was performed using FDS. In simulation, the computational region of a tunnel is 200 m long, 10 m wide. The heat release rate (1 MW to 6 MW) and the height (4 m to 8 m) is changed in the 30 simulation scenarios. Theoretical analysis showed that the dimensionless critical velocity of bifurcation flow only depends on the dimensionless heat release rates, and a mathematical equation is proposed. The reduced-scale experiments indicated that the critical velocity of bifurcation flow is 1.48 times that of critical velocity for preventing backlayering, and the coefficient is in agreement with CFD simulation. A Cross-sectional area of the tunnel (m 2 ) g Acceleration of gravity (m/s 2 ) U Ventilation velocity (m/s) _ Q Heat release rate (kW) u b Critical velocity of bifurcation flow (m/s) u c Critical velocity for preventing backlayering (m/s)