Based　on　the　theory　of　limit　equilibrium,　a　stress　analysis　was　presented　to　study　the　water　burst　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　water　inrush　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　increase　of　tunnel　radius,　increases　with　the　increase　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,　water　inrush　is　likely　to　occur.　Water　inrush　is　more　possible　to　occur　with　increasing　obliquity　of　disruption　zone　and　minishing　angle　of　internal　friction.