A　label-free　hyperbranched　rolling　circle　amplification　(HRCA)　based　fluorescent　sensor　has　been　developed　for　Hg2+　detection.　The　fluorescence　intensity　has　a　linear　relationship　with　the　concentration　of　the　target　in　the　range　of　0.425　pmol　L-1　to　42.5　nmol　L-1,　and　the　detection　limit　is　as　low　as　0.14　pmol　L-1　(S/N　=　3).　The　proposed　sensor　had　been　applied　to　detect　Hg2+　in　water　samples　with　satisfying　results.