Abstract: The smoke spread in the tunnel under longitudinal ventilation conditions was studied by numerical methods, and the smoke stratification and temperature distribution under lower ventilation velocity at upstream and downstream of the fire source were investigated. Based on this, whether the longitudinal smoke control system is suitable for the congested city tunnel was discussed. The research results show that if the tunnel is supplied with longitudinal ventilation at a lower speed in the early stage of the fire, the smoke layer at the downstream of the fire is kept to ensure the safety evacuation of the people. For the fire power of 5 MW, the suitable ventilation speed is about 1.0 m/s; when the fire power is 20 MW, the ventilation speed of 1.0-1.5 m/s meets the requirements. For the fire power of 30 MW, the ventilation speed is 1.5 m/s. When the fire power is large, people near the downstream of the fire source should be evacuated as soon as possible to ensure the safety.