Abstract: Based on the theory of limit equilibrium,a stress analysis was presented to study the waterburst mechanism in normal geologyical condition and fault disruption zone during crossing undersea tunnel,then critical water inrush pressure formula of undersea tunnel was given.Finally,a calculation and waterinrush risk forecast is made in two deep weathered rocks and sections of Xiang'an Tunnel.In conclusion,in normal geologyical conditon,critical water pressure decreases with the increasing of tunnel radius,increases with the increasing of uniaxial compressive strength of surrounding rocks and depth of the tunnel respectively;in fault disruption zone,when the obliquity of disruption zone is bigger than or equal to angle of internal friction,waterinrush is likely to occur.And waterinrush is more possible to occur with increasing obliquity of disruption zone and minishing angle of internal friction.