Abstract: The external water pressures of tunnel lining structures and the limited discharge standards are the focus of tunnel construction in karst mountainous areas. Based on a water-rich tunnel, by using the FLAC^3D software, the reasonable values of the external water pressures for the lining structures of the tunnel and the limited discharge standards are studied through the numerical analysis of fluid-solid coupling. The variation laws of water pressures of the tunnel with displacement without grouting is analyzed, then the values of the external water pressures of the tunnel and the standards of groundwater discharge based on the structural safety are proposed. At the same time, the variation laws of the external water pressures of the tunnel with grouting ring thickness and groundwater discharge under grouting are analyzed, the standards of groundwater discharge and the reasonable values of grouting ring thickness under different grouting thicknesses are put forward.