To solve the air pollution in the process of slagging in the high-altitude construction tunnel and improve the effect of ventilation, a high-altitude tunnel gas diffusion model was constructed based on the changes in environmental parameters at altitude and the theory of turbulence diffusion. Taking a tunnel of Sichuan-Tibet Railway at an altitude of 3 200 m as the research background, the environmental parameters and carbon monoxide （CO）concentration in the tunnel were measured. A construction tunnel slagging model was established by SolidWorks and ANSYS softwares. The component transport equation in Fluent was used to dynamically simulate the law of harmful gas migration and mass concentration distribution at different altitudes. The results show that when the internal combustion engine is operated in the tunnel, the CO distribution is uneven and the range is large near the tunnel working face head-on. Near the exit of the tunnel, the CO distribution gradually stabilized. The CO mass fraction in the tunnel increases with the increase of altitude, but the CO mass concentration shows an opposite trend. From 0 m to 6 000 m, the CO mass fraction increased by 96%, but the influence of environmental parameter changes on the CO mass concentration is more important than the CO emissions, resulting in a decrease of 18% in the CO mass concentration. As a result, the air demand increases non-linearly with altitude while keeping the CO mass fraction constant in the tunnel. According to the obtained air demand calculation model, when the altitude is 3 200 m, the air demand is about 4.95 m3/（kW·min）.