Abstract: The rock surrounding an underground water-sealed cave is a kind of low permeability medium. Its gas permeability property and the change in gas permeability when subjected to different confining pressures draw considerable attention in the construction of underground water-sealed caves. This paper conducts experiments on gas permeability and gas slippage effects, and uses a scanning electron microscope (SEM) to study the weathered granitic gneiss of an underground water-sealed cave in China. The results show that the laws governing the change in gas permeability and rock porosity vary at different stages of confining pressure. It is found that there is a threshold point for confining pressure. The relationship between gas permeability and confining pressure is exponential when the pressure is larger than the value, otherwise, it is a quadratic function relationship. The SEM tests indicate that the different behaviors of gas permeability and porosity are caused by the micro-fissures and micro-void generated by unconformable contact when pressure is applied. Additionally, the measured values of gas permeability need to be modified due to the existence of gas slippage effects, and these must be taken into account when designing and running underground water-sealed caves. Gas slippage is closely related to confining pressure and its maximum impact occurs at a confining pressure of 8 MPa~9 MPa for this weathered granitic gneiss.