Abstract: The diameter, air pressure and air volume of the air duct are limited by the size of the tunnel excavation section, which may lead to the untimely gas dilution in the construction of long-distance tunnels with extra-small section.Therefore, it is necessary to study the effect of hydraulic tunnel ventilation on gas dilution.Relying on the Tingzikou Irrigation District Project, based on CFD numerical simulation method and field measurement, the ventilation effect of tunnel with extra-small section and the air outlet of 15 m away from the tunnel face was simulated and analyzed.The results show that: ① After the tunnel airflow collides with the tunnel face, a large-scale and strong eddy current is formed, and the periphery of the eddy current is the place where gas is easy to accumulate, which hinders the discharge of gas.② The distance between tunnel air flow and tunnel face is 10 m when air flow reaches a stable flow state after coming out of the air duct outlet.The distance between tunnel air flow and tunnel face is 12 m when the gas concentration gradually tends to be stable after dilution and diffusion under the action of air flow.③ Through field detection, when gas gushes from the working face point, the vault of working face is the place where gas is most likely to accumulate, which is roughly consistent with the numerical simulation results.