Abstract: The safety of gas transmission tunnel is very important for the long-term safe operation of gas transmission pipelines, and the seepage safety is very critical for the tunnel undercrossing the river. In this paper, the seepage characteristics were analyzed for a tunnel in Sichuan-to-East Gas Pipeline crossing Jialing River by establishing a full-scale three-dimensional numerical model, and the water inflow was assessed accordingly. Firstly, the permeability distribution characteristics of the site rock mass and the permeability coefficient characteristics of the seepage channels such as the fault fracture zone were obtained by analyzing the engineering geological conditions of the study area. Then, the boundary range of numerical model was determined with consideration to the integrity and independence of groundwater system according to the hydrogeological characteristics. The numerical simulation results show that the fault fracture zone is the main water inflow section with the permeability increasing significantly. Thus, fault grouting could effectively reduce the water inflow and improve the safety of tunnel excavation. Generally, the research results could providea a reference for similar tunnels undercrossing the river.