In　order　to　study　the　seismic　response　characteristics　of　the　giant-span　flat　cavern　(GSFC),　a　nonlinear　finite　element　model　of　the　GSFC　was　firstly　established　by　ABAQUS　software　to　investigate　the　seismic　deformation　characteristics　and　stress　variation　law　of　the　GSFC　in　detail.　Secondly,　the　bearing　mechanism　of　the　GSFC　was　systematically　analyzed.　Finally,　the　damage　evolution　law　and　failure　mechanism　of　the　GSFC　were　revealed.　The　results　showed　that　the　deformation,　stress　and　damage　of　the　GSFC　under　design　and　rare　earthquakes　were　within　safe　and　controllable　limits.　The　interaction　between　the　linings　had　a　certain　damping　and　peak　-elimination　effect.　When　the　PGA　was　0.2　g,　the　peak　acceleration　of　the　inside　lining　was　reduced　by　10%　compared　to　the　outside　lining,　the　deformation　response　was　reduced　by　39%,　and　its　seismic　stress　distribution　was　more　uniform.　In　the　bearing　system　of　the　GSFC,　the　surrounding　rock　was　supported　the　lining　so　that　its　self-bearing　capacity　was　fully　utilized,　while　the　lining　was　embedded　by　the　surrounding　rock,　improving　its　integrity　and　stiffness.　Under　extreme　earthquakes,　the　damage　evolution　processes　of　the　surrounding　rock　and　the　lining　were　coupled　with　each　other.　The　continuous　plastic　strain　occurred　mainly　at　the　bottom　and　top　of　the　surrounding　rock.　The　outside　lining　was　mostly　destroyed,　and　the　inside　lining　gradually　degraded　into　a　double-hinged　arch　structure　supported　by　the　side　walls,　so　that　the　seismic　performance　of　the　surrounding　rock-lining　bearing　system　was　lost.　The　degraded　structure　can　maintain　a　relatively　stable　vertical　self　-equilibrium　state　and　is　not　prone　to　extreme　damage　such　as　overall　collapse.　The　research　conclusions　could　provide　a　theoretical　basis　and　scientific　reference　for　accurate　seismic　prediction　and　safety　evaluation　of　the　GSFC.