To make the longitudinal ventilation systems more efficient and energy saving in road tunnels, it is being combined with typical model predictive control (MPC) strategies. Good application of such method is based on a comprehensive understanding of the system characteristics. Using CFD (Computational Fluid Dynamics) simulation, step response model is chosen to describe the dynamic behaviors of the system in a road tunnel with comprehensive health-related indexes which is difficult to obtain by field measurement. The results show that the "Fan Model", which characterizes the effect of jet speed on CO (carbon monoxide) concentration, is approximately bilinear within fan's economical working range and system settling time is longer when jet speed decreases than it increases. The "Tunnel tube model", which characterizes the effect of traffic intensity on CO concentration, links to traffic flow features, and has a similar settling time in normal operation and a traffic-speed-related delay time when maximum CO concentration is considered. This can provide guidance or index to control the tunnel ventilation system economically and useful data to implement MPC strategy in the system.