Ensuring　the　safety　of　undersea　shield　tunnels　constructed　in　soft　marine　deposits　that　are　subject　to　strong　seismic　motion　represents　a　major　engineering　challenge.　An　example　of　one　such　undersea　shield　tunnel　is　the　2.70　km-long　subsea　highway　tunnel　crossing　under　the　Gulf　of　Suai　in　the　Rongjiang　River　Estuary　in　Shantou,　China.　Using　the　generalized　response　displacement　method,　the　authors　developed　a　nonlinear　seismic　response　analysis　of　the　segmental　lining　for　the　2.7　km-long　Suai　tunnel.　In　the　proposed　longitudinal　seismic　response　analysis,　the　engineering　geology　characteristics　and　nonlinear　dynamic　behavior　of　the　Suai　seabed　soil,　the　nonuniform　mesh　layout　of　the　free-field　site,　the　artificial　boundary　conditions　and　nonuniform　seismic　input,　simulation　model,　and　the　parameters　of　soil-tunnel　interaction　systems,　are　considered　in　detail.　Special　emphasis　is　given　to　the　irregular　unloading-reloading　rules　for　the　stress-strain　hysteresis　loop,　the　seismic　input　at　artificial　boundary　nodes,　and　the　spatial　incoherency　of　ground　motions　in　the　seabed　site.　The　opening　width　at　the　ring　intersegment　under　simultaneous　actions　of　longitudinal,　transversal,　and　vertical　seismic　motions　is　critical　for　seismic　safety.　The　proposed　methodology　was　deemed　conservative,　as　demonstrated　by　a　comparison　with　dynamic　transient　analysis　using　the　three-dimensional　finite　element　method.　The　results　of　this　pilot　study　should　be　of　use　in　the　construction　of　future　long　subsea　shield　tunnels　in　the　high　seismic　intensity　region.