Electrochemiluminescence　(ECL)　of　graphitic　carbon　nitride　nanosheets　(CNNSs)　is　showing　extensive　interest　to　researchers.　However,　the　poor　ECL　stability　of　CNNSs　has　always　been　an　urgent　problem,　which　severely　limits　its　application.　Herein,　a　systemic　investigation　is　carried　out　for　the　exact　mechanism　affecting　the　ECL　stability　of　CNNSs.　It　is　found　experimentally　that　dissolved　oxygen　in　the　solution　is　selectively　reduced　into　H2O2　through　a　two-electron　pathway　on　the　surface　of　CNNSs.　The　enriched　electrogenerated　H2O2　on　the　electrode　surface　leads　to　the　progressive　inhibition　of　ECL　of　CNNSs　due　to　its　competition　with　negatively　charged　CNNSs　(CNNSs　center　dot-)　for　sulfate　radical　ions　(SO4　center　dot-).　On　the　basis,　the　pathway　of　eliminating　O2　by　N2,　composing　CNNSs　with　other　nanomaterials　to　realize　the　four-electron　reduction,　or　improving　the　concentration　of　K2S2O8　is　proved　to　be　effective　means　for　improving　ECL　stability　successfully.　Proofing　the　concept　of　CNNSs　with　high　ECL　stability,　an　ECL　sensor　is　developed　for　the　detection　of　H2O2.　This　work　not　only　exploits　the　intrinsic　mechanism　of　poor　ECL　stability　of　CNNSs,　but　also　provides　efficient　methods　to　improve　the　ECL　stability.　Thus,　we　believe　it　has　promising　attraction　in　the　research　of　CNNSs-based　assays.