A　simple,　fast,　sensitive,　and　homogeneous　electrochemical　sensor　based　on　the　T-Hg2+-T　structure　and　exonuclease　III-assisted　recycling　amplification　has　been　constructed　for　mercury　ion　(Hg2+)　detection.　The　cT　and　methylene　blue-labeled　DNA　probes　(MB-TDNA)　were　designed　to　contain　poly　T　sequences,　which　were　repulsed　from　the　negatively　charged　indium　tin　oxide　(ITO)　electrode　due　to　their　abundant　negative　charges.　Hg2+　could　trigger　the　formation　of　double-stranded　DNA　(dsDNA)　between　two　DNA　probes　owing　to　the　stable　T-Hg2+-T　structure.　Then,　Exo　III　specifically　recognizes　the　cleavage　of　the　double-stranded　structure　to　release　a　methylene　blue-labeled　mononucleotide　fragment　(MB-MF).　Moreover,　the　release　of　the　target　Hg2+　induces　new　hybridization　and　produces　a　large　number　of　MB-MFs;　MB-MFs　are　not　repulsed　by　the　negatively　charged　ITO　electrode　surface,　thus　producing　a　significant　current　signal.　Under　optimal　conditions,　the　differential　pulse　voltammetric　(DPV)　response　had　a　linear　relationship　with　the　logarithm　of　Hg2+　concentration　in　the　range　of　1.0　nM-0.5　M,　and　the　proposed　method　displayed　great　applicability　for　detecting　Hg2+　in　tap-water　samples.