AbstractWith the progressive rise in energy demand, the scale of oil and gas pipelines continues to increase. Tunnels have been developed to plan the space reasonably and to reduce pipeline maintenance and repair difficulties. However, for natural gas pipes laid in the tunnel, leakage accidents can cause severe results. Based on the computational fluid dynamics (CFD) theory, the leakage and diffusion characteristics of natural gas in tunnels under various scenarios are analyzed using flame acceleration simulation (FLACS) software. It can be concluded that (1) natural gas leakage in the tunnel can result in the occurrence of self-suffocation, where small-hole leaks pose a greater threat than large-hole leaks, which clearly differs from leaks into open rooms; (2) the larger the leakage diameter, the shorter the tunnel and the higher the chance of self-suffocation happening; (3) the flammable gas mass in a tunnel depends on several factors, including gas leak rate, gas mixing rate, diffusion rate between gas with higher concentrations and gas with much lower concentrations, and the gas diffusion rate as it exits the tunnel; (4) when a leakage hole is in the middle of the tunnel and the leakage direction is upward, there is the greatest danger; and (5) for the same tunnel length, the larger the leakage diameter, the smaller the flammable gas mass. The study presents the characteristics of natural gas diffusion in a tunnel and offers suggestions for leakage accident emergency repair.