The　overexpression　of　microRNA-21　(miRNA-21)　is　closely　associated　with　gastric　cancer　(GC)　and　may　be　used　as　a　biomarker　to　diagnose　GC　early.　Here,　a　colorimetry　and　fluorescence　dual-mode　biosensor　for　the　detection　of　miRNA-21　was　developed　from　polydopamine-coated　magnetic　upconversion　nanoparticles　(Fe3O4@UCNP@PDA)　and　aptamer-modified　gold　nanoparticles　(AuNP-apt).　The　strong　adsorption　of　singlestranded　DNA　by　PDA　results　in　close　proximity　between　the　energy　donors　(UCNPs)　and　acceptors　(AuNPs),　resulting　in　luminescence　quenching　of　the　UCNPs.　The　miRNA-21　can　hybridize　with　the　apt-conjugated　AuNPs,　and　the　AuNPs　move　away　from　the　UCNPs,　which　results　in　the　luminescence　recovery　of　the　UCNPs　and　the　catalytic　activity　of　the　AuNPs.　After　H2O2　and　tetrapaminophenylethylene　were　added,　the　yellow　solution　turned　green　and　gradually　deepened　with　increasing　miRNA-21　concentration.　Under　optimal　conditions,　the　detection　limits　of　miRNA-21　by　fluorescence　and　colorimetry　assays　are　8.90　pM　and　185　pM,　respectively.　In　addition,　the　developed　approach　was　utilized　to　measure　miRNA-21　in　serum　samples,　and　the　recovery　rates　ranged　from　105.4-107.1%　and　98.5-109.3%,　respectively.　The　above　results　indicate　great　promise　for　the　early　diagnosis　of　GC　because　of　its　high　sensitivity　and　selectivity.