Underground utility tunnels, as a critical component of urban infrastructure, integrate various pipelines such as water supply, electricity, and natural gas, thereby significantly enhancing the utilization efficiency of underground space and reducing road excavation. However, once a natural gas pipeline leaks within the tunnel, its flammable and explosive nature poses a severe threat to the safety of the tunnel and surrounding facilities. Therefore, studying the diffusion patterns of natural gas leaks in utility tunnels is crucial for ensuring the safe operation of these tunnels and urban systems. This study employs the ANSYS Fluent software to construct a three-dimensional model of a 200-meter-long gas compartment. The Realizable k-ε turbulence model is applied to simulate the diffusion process of natural gas leaks under different ventilation conditions and leak locations. Simulations were performed with varying exhaust velocities (no exhaust, 1.5 m/s, and 2.5 m/s) and different leak locations (5 m, 25 m, 50 m, 75 m, 100 m, and 125 m from the air inlet) to analyze the distribution of methane within the tunnel over a 200-second period following a leak. The results show that: Exhaust velocity significantly influences the diffusion of methane gas. As the ventilation speed increases, the diffusion range and peak concentration of the gas decrease markedly, particularly under the 2.5 m/s ventilation condition, where the duration and extent of high-concentration areas are substantially reduced. Additionally, the location of the leak plays a pivotal role in the gas diffusion characteristics. Leaks closer to the air inlet exhibit faster diffusion and a more gradual concentration gradient, whereas leaks farther from the inlet tend to form high-concentration accumulation zones, increasing safety risks.