A　kind　of　coumarin-modified　gold　nanoparticle　by　the　bridge　of　dsDNA　chains　was　designed　and　synthesized　for　sensitive　detection　of　DNase　I.　The　fluorescence　of　coumarin　343　at　emission　wavelengths　of　491　nm　excited　at　440　nm　was　quenched　by　the　gold　nanoparticles　due　to　the　energy　transfer　process　after　the　coumarin　343　was　connected　on　the　gold　nanoparticles　by　DNA　chains.　When　dsDNA　chains　were　cut　off　by　DNase　I,　the　coumarin　343　molecules　were　released　from　gold　nanoparticles　and　the　fluorescence　of　coumarin　343　would　be　restored.　The　DNase　I　activity　could　be　detected　by　this　fluorescence　assay　with　a　high　sensitivity　based　on　the　change　of　the　energy　transfer　efficiency.　The　intensity　of　restored　fluorescence　is　linearly　related　to　the　quantity　of　DNase　I　in　the　range　from　1.0　to　40　mU/mL　with　a　detection　limit　of　0.22　mU/mL.　This　design　idea　could　render　a　useful　way　to　develop　similar　molecular　or　enzyme　sensor　in　analytical　or　biological　fields.