Currently,　drug-induced　liver　injury　(DILI)　has　become　a　huge　concern　for　the　majority　of　modern　medicine,　whereas　the　diagnosis　of　DILI　is　still　in　its　infancy　due　to　the　lack　of　appropriate　methods.　Herein,　based　on　the　fact　that　nitric　oxide　(NO)　has　been　recognized　as　an　early　unifying,　direct,　and　vital　biomarker　for　DILI,　we　rationally　designed　and　developed　a　NO-responsive　ratiometric　fluorescent　nanoprobe　DCNP@MPS@IR　NO　to　quantitatively　detect　NO　and　monitor　DILI　in　the　second　near-infrared　(NIR-II)　window.　In　the　presence　of　NO,　due　to　the　conversion　of　IR　NO　into　IR　RA　and　excellent　stability　of　the　downconversion　nanoparticle　(DCNP),　DCNP@MPS@IR　NO　could　present　a　＂Turn-On＂　fluorescence　signal　at　1050　nm　under　808　nm　excitation　(F-1050　(Em,)　(808)　(Ex))　and　an　＂Always-On＂　fluorescence　signal　at　1550　nm　under　980　nm　excitation　(F-1550　Em,F-　(980　Ex)),　which　led　to　a　＂Turn-On＂　ratiometric　fluorescence　signal　F-1050(　Em,　808　Ex)/F-1550　Em,F-　980　Ex.　DCNP@MPS@IR　NO　was　then　successfully　applied　in　vitro　to　selectively　detect　NO,　at　a　linear　concentration　range　of　0-100　mu　M　with　a　limit　of　detection　of　0.61　mu　M.　In　vivo　results　revealed　that　DCNP@MPS@IR　was　available　to　quantify　NO　in　acetaminophen　(APAP)-induced　liver　injury,　monitor　DILI,　and　screen　an　antidote　for　APAP　through　NIR-II　ratiometric　fluorescence　imaging.　We　envision　that　our　nanoprobe　DCNP@MPS@IR　NO　might　become　a　really　useful　biotechnology　tool　for　visualizing　and　early　diagnosis　of　drug-induced　liver　injury　and　revealing　the　mechanism　of　drug　hepatotoxicity　in　the　clinic　in　the　near　future.