Abstract: To ensure the safe operation of railway tunnels and prevent gas disasters in auxiliary tunnels, a study is conducted based on the closure of a gas-containing auxiliary tunnel (inclined shaft) in a certain railway tunnel. Using computational fluid dynamics (CFD) software for numerical simulation, this study reveals the gas concentration distribution characteristics as well as migration and accumulation patterns under different closure conditions. The results show that the highest gas concentration occurs near the coal seam. Due to the slope and the gas′s own properties, after the gas escapes from the coal seam, it first accumulates at the top and then diffuses downward under the influence of the concentration gradient. Gas concentration is higher in the uphill section than in the downhill section.When only the lower end is sealed, or both ends are open, the gas concentration remains low but eventually reaches0.05-0.16, close to the explosive limit. In other sealing conditions, the final gas concentration is generally higher than 0.16, exceeding the explosive limit. Sealing the upper end and setting a pressure relief hole at the upper end has limited effects, resulting in generally higher final gas concentrations. It is recommended to strengthen monitoring and ventilation in areas with excessively high gas concentrations in the uphill section and seal the lower end of the inclined shaft to prevent gas from leaking into the main railway tunnel.