Rare　earth　up-conversion　nanoparticles　(UCNPs)　have　been　used　to　construct　fluorescent　nanosensors　because　of　their　low　toxicity,　good　chemical　stability,　and　low　background　fluorescence.　Core-shell　UCNPs　were　prepared　using　the　solvothermal　method,　and　water-soluble　core-shell　UCNPs　(Cit-CS-UCNPs)　were　obtained　by　surface　ligand　exchange　with　sodium　citrate.　The　Cit-CS-UCNPs　were　used　as　the　energy　donor　of　the　fluorescence　sensor,　and　manganese　dioxide　(MnO2)　nanosheets　were　used　as　the　energy　receptor　of　the　fluorescence　sensor.　Based　on　the　fluorescence　resonance　energy　transfer　(FRET)　mechanism,　a　fluorescence　nanosensor　(Cit-CS-UCNPs-MnO2)　was　constructed　for　the　detection　of　hydrogen　dioxide　(H2O2)　and　tert-butylhydroquinone　(TBHQ　)　as　food　additives.　The　prepared　nanomaterials＇　morphology,　structure,　and　properties　were　characterized　by　scanning　electron　microscopy　(SEM),　fluorescence　spectrum,　and　ultraviolet　spectrum　(UV-vis).　The　effects　of　quenching　agent　concentration,　incubation　temperature,　and　incubation　time　on　the　detection　performance　of　the　fluorescence　sensing　system　were　investigated.　According　to　the　fluorescence　spectra　and　UV-Vis　experimental　results,　the　maximum　emission　peak　of　Cit-CS-UCNPs　is　654　nm.　After　the　combination　of　Cit-CS-UCNPs　and　MnO2,　core-shell　UCNPs　undergo　fluorescence　quenching.　When　H2O2　is　present,　the　fluorescence　of　Cit-CS-UCNPs　recovers.　The　results　indicate　that　H2O2　and　MnO2　nanosheets　undergo　redox　reaction　at　this　band,　and　MnO2　nanosheets　are　reduced　to　Mn2+,　which　gradually　dissociates　from　the　surface　of　Cit-CS-UCNPs.　In　the　presence　of　TBHQ,　the　peak　of　the　Cit-CS-UCNPs-MnO2　and　TBHQ　system　was　shifted　to　253　nm,　indicating　that　the　redox　reaction　occurred　between　TBHQ　and　MnO2　nanosheets,　and　the　FRET　effect　was　reduced　in　the　Cit-CS-UCNPs-MnO2　system,　and　the　fluorescence　intensity　increased.　It　can　be　seen　from　the　SEM　results　that　MnO2　nanosheets　are　uniformly　coated　around　Cit-CS-UCNPs,　and　maintain　good　dispersion　in　water,　indicating　that　MnO2　nanosheets　are　modified　on　the　surface　of　Cit-CS-UCNPs.　The　concentration　of　the　quencher　potassium　permanganate　(KMnO4)　was　optimized,　and　the　results　showed　that　when　the　concentration　of　KMnO4　is　10　mol.L-1,　the　quenching　efficiency　can　reach　90%.　The　detection　conditions　were　optimized,　and　the　results　showed　that　when　the　incubation　time　of　H2O2　was　25　min,　the　redox　reaction　between　MnO2　and　H2O2　was　complete.　The　fluorescence　recovery　value　of　Cit-CS-UCNPs-MnO2　was　the　maximum.　The　incubation　time　of　TBHQ　was　30　min.　Under　the　optimal　experimental　conditions,　the　fluorescence　intensity　of　Cit-CS-UCNPs-MnO2　has　a　good　linear　relationship　with　the　concentration　of　H2O2　(0　similar　to　1　000　mu　mol.L-1)　and　TBHQ　(0　similar　to　0.6　mmol.L-1).　The　optimal　experimental　conditions　were　maintained,　and　representative　metal　ions　(such　as　K+,　Na+,　Ca2+　and　Mg2+)　and　common　food　additives　(BA,　Glu,　PS,　Suc,　Nat　and　Ino)　in　food　were　selected　as　research　objects.　The　results　showed　that　compared　with　H2O2,　Cit-CS-UCNPs-MnO2　did　not　react　strongly　to　other　added　substances,　and　the　overall　fluorescence　signal　of　the　sensor　did　not　fluctuate　much.　Therefore,　it　can　be　seen　that　Cit-CS-UCNPs-MnO2　can　detect　the　specificity　of　H2O2　and　TBHQ.