Subsea　tunnel　lining　structures　should　be　designed　to　sustain　the　loads　transmitted　from　surrounding　ground　and　groundwater　during　excavation.　Extremely　high　pore-water　pressure　reduces　the　effective　strength　of　the　country　rock　that　surrounds　a　tunnel,　thereby　lowering　the　arching　effect　and　stratum　stability　of　the　structure.　In　this　paper,　the　mechanical　behavior　and　shape　optimization　of　the　lining　structure　for　the　Xiang＇an　tunnel　excavated　in　weathered　slots　are　examined.　Eight　cross　sections　with　different　geometric　parameters　are　adopted　to　study　the　mechanical　behavior　and　shape　optimization　of　the　lining　structure.　The　hyperstatic　reaction　method　is　used　through　finite　element　analysis　software　ANSYS.　The　mechanical　behavior　of　the　lining　structure　is　evidently　affected　by　the　geometric　parameters　of　crosssectional　shape.　The　minimum　safety　factor　of　the　lining　structure　elements　is　set　to　be　the　objective　function.　The　efficient　tunnel　shape　to　maximize　the　minimum　safety　factor　is　identified.　The　minimum　safety　factor　increases　significantly　after　optimization.　The　optimized　cross　section　significantly　improves　the　mechanical　characteristics　of　the　lining　structure　and　effectively　reduces　its　deformation.　Force　analyses　of　optimization　process　and　program　are　conducted　parametrically　so　that　the　method　can　be　applied　to　the　optimization　design　of　other　similar　structures.　The　results　obtained　from　this　study　enhance　our　understanding　of　the　mechanical　behavior　of　the　lining　structure　for　subsea　tunnels.　These　results　are　also　beneficial　to　the　optimal　design　of　lining　structures　in　general.