Due　to　the　strong　biological　penetration,　large　stoke　shift,　good　photostability　and　biocompatibility,　up-conversion　nanoparticles　(UCNPs)　were　widely　used　in　biomedical　analysis.　In　this　study,　OA-UCNPs,　which　are　UCNPs　with　oleic　acid　(OA)　as　ligand,　were　modified　by　polyacrylic　acid　(PAA)in　order　to　prepare　hydrophilic　nanoparticles　PAA-UCNPs,　and　then　aptamers　(Apt)　were　covalently　coupled　to　the　surface　of　PAA-UCNPs　by　amidation　reaction　to　obtain　Apt-UCNPs.　A　method　for　specific　detection　of　sulfadimethoxine　(SDM)was　constructed　by　using　Apt-UCNPs　as　energy　donors　and　black　hole　quencher　(BHQ1)　as　an　energy　acceptor　fluorescence　resonance　energy　transfer.　The　structure　and　performance　of　UCNPs,　PAA　and　PAA-UCNPs　were　characterized　by　infrared　spectroscopy　(FTIR)　and　scanning　electron　microscope　(SEM).　The　results　of　FTIRshowed　that　compared　with　UCNPs,　PAA-UCNPs　appeared　at　a　new　peak　at　1　722　cm　which　was　attributed　to　the　-C=O　stretching　vibration　peak　of　PAA,　and　the　wide　band　around　3　400　cm(-1)　attributed　to　the　O-H　stretching　vibration　of　PAA;　the　results　of　the　SEM　test　indicated　that　the　size　of　UCNPs　and　PAA-UCNPs　was　31　and　49　nm,　respectively.　It　may　be　because　the　molecular　volume　of　long-chain　PAA　is　larger　than　that　of　oleic　acid,　so　coating　on　the　surface　of　UCNPs　will　increase　its　size,　and　the　above　results　indicated　PAA　may　have　had　been　modified　to　the　surface　of　UCNPs.　The　structure　and　properties　of　Apt-UCNPs,　Apt　and　PAA-UCNPs,　were　characterized　by　ultraviolet-visible　spectroscopy.　It　was　found　that　compared　with　PAA-UCNPs,　Apt-UCNPs　had　a　more　obvious　Apt　characteristic　absorption　peak　at　260　nm,　which　showed　that　the　aptamer　could　have　been　modified　to　the　surface　of　UCNPs.　In　addition,　the　mechanism　of　using　Apt-UCNPs　to　detect　SDM　was　also　discussed.　The　results　showed　that　the　emission　peak　of　Apt-UCNPs　at　540　nm　overlapped　with　the　absorption　peak　of　BHQ1,　which　indicated　that　the　energy　on　Apt-UCNPs　can　be　transferred　to　BHQ1　through　the　fluorescence　resonance　energy　transfer　effect　so　that　the　fluorescence　of　Apt-UCNPs　could　be　quenched.　The　concentration　of　BHQ1　in　the　detection　system　was　optimized　and　the　results　showed　that　when　the　concentration　of　BHQ1　was　15　mu　mol.L-1,　the　fluorescence　quenching　efficiency　was　as　high　as　55%.　Under　the　best　experimental　conditions,　there　was　a　good　linear　relationship　between　the　relative　fluorescence　intensity　and　the　SDM　concentration　(150　similar　to　1　000　ng.mL(-1)).　The　analogues　of　SDM　(sulfapyridine　and　sulfacetamide)　were　selected　as　control　experiments.　It　was　found　that　although　the　concentration　of　sulfapyridine　and　sulfacetamide　reached　500　ng.mL(-1),　the　fluorescence　intensity　recovery　in　the　system　was　less,　which　indicated　that　the　detection　method　could　specifically　recognize　SDM.