Ceiling beams at the top of tunnels are more common in actual projects. Under the influence of thermal buoyancy, the ceiling structure significantly affects the diffusion characteristics of fire smoke within the tunnel. This study investigated the influence of ceiling structural characteristics (beam height (h_B) and beam spacing (d_B)) on tunnel longitudinal ventilation through numerical simulation. The results show that the spacing between tunnel ceiling beams has negligible impact on the critical velocity (V), and the determination of the critical velocity is primarily correlated with the height of the ceiling beams. Moreover, it established a dimensionless critical velocity (V_c) model for the tunnels with multiple beams in the ceiling, and this model is suitable for predicting the critical longitudinal velocity of tunnels with ceiling beams whose dimensionless beam height is less than 0.25. When the dimensionless beam height exceeds 0.25, the predictive values of this model are excessively high. This study broadens the application scope of fire smoke control models, which can offer technical support for the design of smoke prevention and exhaust systems in tunnels with similar structures.