MicroRNAs　(miRNAs),　a　class　of　small　molecules　with　important　regulatory　functions,　have　been　widely　used　in　biosensing　as　biomarkers　for　the　early　diagnosis　of　various　diseases　(such　as　osteoporosis).　Therefore,　it　is　necessary　to　develop　a　miRNA　detection　platform　with　high　sensitivity　and　specificity.　Herein,　an　upconversion　biosensor　based　on　fluorescent　resonant　energy　transfer　(FRET)　for　the　detection　of　miRNAs　was　developed.　Gold　nanoparticles　(AuNPs,　fluorescence　acceptors)　were　conjugated　with　aptamers,　while　upconversion　nanoparticles　(UCNPs,　fluorescence　donors)　were　functionalized　with　the　corresponding　complementary　DNA　(cDNA).　The　upconversion　luminescence　(UCL)　is　quenched　vis　FRET　due　to　the　good　overlap　between　UCNP　fluorescence　emission　and　AuNP　absorption.　In　the　presence　of　the　target　miRNA,　the　aptamers　preferentially　bind　to　the　miRNA,　thereby　dissociating　the　UCNP-cDNA　from　the　AuNP-aptamers,　resulting　in　the　recovery　of　the　UCL.　The　UCL　recovery　efficiency　increased　in　a　miRNA　concentration-dependent　manner,　which　supported　the　principle　of　miRNA　quantification.　The　UCNP-based　FRET　biosensor　exhibited　sensitive　detection　of　miRNA-21　(as　a　model　analyte)　with　low　limit　of　detection　of　5.91　pM.　In　addition,　depending　on　the　near-infrared　excitation　and　a　simple　complementary　binding　reaction　during　the　detection　process,　the　designed　biosensor　not　only　effectively　eliminated　the　autofluorescence　but　also　reduced　the　detection　time.