Typical　fluorescent　biosensors　use　fluorescently　labeled　ssDNA　for　target　recognition　and　nanomaterials　for　signal　transduction.　Herein,　we　propose　a　reverse　sensing　strategy　that　Mo5N6　nanosheets　are　used　for　target　recognition　while　fluorescein　(FAM)-labeled　ssDNA　only　serves　for　signal　generation.　We　discover　that　Mo5N6　nanosheets　show　high　fluorescence　quenching　ability　(＞95%)　and　selective　recognition　for　sodium　hexametaphosphate　(SHMP).　After　FAM-labeled　ssDNA　is　adsorbed　on　Mo5N6　nanosheets,　the　fluorescence　is　quenched　due　to　the　photoinduced　electron　transfer　(PET)　effect　between　FAM　and　Mo5N6　nanosheets.　SHMP　can　specifically　displace　the　adsorbed　FAM-labeled　ssDNA　from　Mo5N6　nanosheets,　resulting　in　more　than　80%　fluorescence　recovery　on　addition　of　5　mu　mol　L-1　SHMP.　This　biosensor　can　sensitively　detect　SHMP　down　to　150　nmol　L-1　and　selectively　recognize　SHMP　over　glucose,　lactose,　common　amino　acids,　Zn2+,　Mg2+,　Ca2+　and　other　phosphates　(such　as　Na2HPO4,　sodium　pyrophosphate,　sodium　tripolyphosphate).　This　biosensor　also　shows　great　potential　for　the　detection　of　SHMP　in　bacon　sample.　This　work　not　only　provides　a　facile　sensitive　and　selective　biosensor　for　SHMP　but　also　exploits　the　application　of　transition　metal　nitrides　in　the　field　of　sensing　and　biosensing.