Abstract: In order to study the influence of the air velocity on the smoke distribution velocity and distance in the roadway in the process of the fire, this paper analyzed the smoke diffusion process in the tunnel. The smoke spread characteristics with different air-speed in the tunnel were simulated and analyzed with Fluent software. The simulation results show that there is a position correlation between CO distribution distance in the return-side tunnel and air-speed. And there is a negative correlation between CO distribution distance in the intake-side tunnel and roadway air velocity. Meanwhile, the CO distribution in the intake-side tunnel seems to stop. And the stopping time and distance decrease as air-speed increases.