A large amount of fissures would occur on tunnel face and tunnel sidewalls during deep long karst tunnel excavation. Due to the interaction of farfield stress and water pressure, the fissures would be expanded and the water flow rate will be increased. As a result, largescale water and mud gushing would occur.The mechanism of internal water effect of fissures is considered to be hydraulic fracture effect and hydrostatic expansion effect popularly. The critical water pressure stresses of tensoshear failure and compressionshear failure in conditions of plane strain are calculated based on fracture mechanics and fluid mechanics. The calculated results show that the critical water pressure stress of tensoshear failure is larger than that of compressionshear failure. The compressionshear failure is the main failure mode of watersaturated fissure.