In　this　paper,　a　novel　and　signal-on　electrochemical　biosensor　based　on　Hg2+-　triggered　nicking　endonucleaseassisted　target　recycling　and　hybridization　chain　reaction　(HCR)　amplification　tactics　was　developed　for　sensitive　and　selective　detection　of　Hg2+.　The　hairpin-shaped　capture　probe　A　(PA)　contained　a　specific　sequence　which　was　recognized　by　nicking　endonuclease　(NEase).　In　the　presence　of　Hg2+,　probe　B　(PB)　hybridized　with　PA　to　form　stand-up　duplex　DNA　strands　via　the　Hg2+　mediated　thymine-Hg2+-thymine　(T-Hg2+-T)　structure,　which　automatically　triggered　NEase　to　selectively　digest　duplex　region　from　the　recognition　sites,　spontaneously　dissociating　PB　and　Hg2+　and　leaving　the　remnant　initiators.　The　released　PB　and　Hg2+　could　be　reused　to　initiate　the　next　cycle　and　more　initiators　were　generated.　The　long　nicked　double　helices　were　formed　through　HCR　event,　which　was　triggered　by　the　initiators　and　two　hairpin-shaped　signal　probes　labeled　with　methylene　blue,　resulting　in　a　significant　signal　increase.　Under　optimum　conditions,　the　resultant　biosensor　showed　the　high　sensitivity　and　selectivity　for　the　detection　of　Hg2+　in　a　linear　range　from　10　pM　to　50　nM　(R-2=0.9990),　and　a　detection　limit　as　low　as　1.6　pM　(S/N=3).　Moreover,　the　proposed　biosensor　was　successfully　applied　in　the　detection　of　Hg2+　in　environment　water　samples　with　satisfactory　results.