A　fluorescent　biosensor　for　ochratoxin　A　was　fabricated　on　the　basis　of　a　new　nanocomposite　(Fe3O4/g-C3N4/HKUST-1　composites).　Fe3O4/g-C3N4/HKUST-1　was　synthesized　in　this　work　for　the　first　time,　which　combined　HKUST-1　with　g-C3N4　to　improve　its　chemical　stability.　Fe3O4/g-C3N4/HKUST-1　composites　have　strong　adsorption　capacity　for　dye-labeled　aptamer　and　are　able　to　completely　quench　the　fluorescence　of　the　dye　through　the　photoinduced　electron　transfer　(PET)　mechanism.　In　the　presence　of　ochratoxin　A　(OTA),　it　can　bind　with　the　aptamer　with　high　affinity,　causing　the　releasing　of　the　dye-labeled　aptamer　from　the　Fe3O4/gC(3)N(4)/HKUST-1　and　therefore　results　in　the　recovery　of　fluorescence.　The　fluorescence　intensity　of　the　biosensor　has　a　linear　relationship　with　the　OTA　concentration　in　the　range　of　5.0-160.0　ng/mL.　The　LOD　of　sensor　is　2.57　ng/mL　(S/N=3).　This　fluorescence　sensor　based　on　the　Fe3O4/g-C3N4/HKUST-1　composites　has　been　applied　to　detect　OTA　in　corn　with　satisfying　results.