In transportation engineering, it has been a focus to evaluate the resistance to water pressure of railway tunnel lining, especially in the southwestern region of China, where the hydrogeological conditions are complicated. When the water around lining does not fully drain away, high water pressure may cause the tunnel lining cracking, which will be a threat to tunnel safety. Therefore, it is very important to evaluate reasonably the anti-water pressure capability of the lining. In this paper, the continuous-discontinuous element method (CDEM) is used to simulate the progressive failure process of the tunnel lining under different water pressures. The relationships among the parameters such as displacement, stress, strain, interface crack factor and interface crack ratio are analyzed. Finally, the relationship between the interface crack ratio and the instability of the lining structure is obtained under the different water pressure conditions. The loading capacity of tunnel lining can be divided into four stages, i.e. safety stage, relative safety stage, critical instability stage and instability stage. Based on the interface crack ratio, a method for evaluating the current state and the loading capacity of tunnel lining is proposed, which is particularly valuable for designing the lining structure under water pressure.