<p>To clarify the influence of downflow fire location on the CO distribution in complex ventilation system, a topological structure model of entire ventilation system was established by on-site measurement and TF1M3D platform. The spatial and temporal distribution characteristics of CO under different downflow fire locations of belt roadway, as well as the disturbance effects of fire on the branch wind volumes, were then studied. The research results show that as the distance between downflow fire location and return shaft decreases, the CO concentration in side branch increases. The appearance time of CO in side branches is later than that in main airway, and the peak CO concentration is 11.74% to 31.56% lower than that in main airway. The dynamic balance relationship between fire-heating wind pressure and fan pressure under different downflow fire locations of main airway affects the migration of CO in the ventilation system. When there is a time lag effect between the peak retrograde wind volume of main airway and the fire source intensity at different downflow fire locations, smoke flow reversal occurs in main airway and its connected branches. By information entropy theory, a CO accumulation risk model considering the stability of ventilation system is established to assess accumulation risk of CO in the roadway under disaster conditions.</p>