The　carbon　nitride　(CN)　with　selenide　(Se)　modification　and　porous　thin　nanosheet　structure　(m-CNNSs)　has　been　successfully　presented　via　an　effective　two-step　continuous　thermal　treatment　method.　The　as-prepared　m-CNNSs　show　a　photocatalytic　H2　generation　and　CO2　reduction　performance　under　visible　light,　the　apparent　quantum　yield　of　H2　generation　(λ　=　420　nm)　reached　8.1%.　This　improved　photocatalytic　performance　originates　from　the　large　surface　areas　and　porous　nanostructure　that　accelerated　separation　of　photoexcited　charge　carriers　and　promoted　mass-transfer　process.　Particularly,　the　formation　of　the　Se　modified　in　the　thin　CN　sheets　further　endow　it　with　reduced　band　gap,　more　exposed　active　edges,　and　extended　visible　light　absorption　range.　This　work　not　only　presents　a　simple　strategy　to　enhance　the　photocatalytic　performance　for　CN,　but　also　opens　ups　a　new　pathway　for　the　rational　preparation　of　effective　polymeric　photocatalysts　by　harnessing　the　synergistic　effects　to　simultaneously　optimize　the　electronic　and　frame　structure　of　polymeric　photocatalysts.　©　2019