In　this　study,　a　novel　electrochemiluminescence　(ECL)　biosensor　based　on　a　CdSe@CdS　quantum　dot　(QD)-functionalized　MoS2-modified　electrode　was　developed　for　the　sensitive　detection　of　mercury　ions.　Polycationic　poly(diallyldimethylammonium　chloride)　(PDDA)　and　negatively　charged　QDs　were　adsorbed　on　the　surface　of　MoS2,　in　sequence,　to　form　the　QD-PDDA-MoS2　composites,　which　were　employed　as　matrices　for　immobilizing　thymine-rich　DNA　sequences　(DNA(1)).　The　gold　nanoparticles　(AuNPs)　were　combined　with　another　type　of　thymine-rich　DNA　sequences　(DNA(2))　and　glucose　oxidase　(GOD)　to　prepare　the　GOD-AuNP-DNA(2)　conjugates.　The　composites　were　characterized　via　UV-visible　absorption　spectroscopy,　fluorescence　and　transmission　electron　microscopy.　A　thymine-Hg2+-thymine　complex　was　formed　through　Hg2+　mismatching　with　DNA(1)　and　DNA(2).　GOD　was,　therefore,　modified　onto　the　electrode　surface.　GOD　catalyzed　the　reduction　of　dissolved　oxygen　by　glucose　to　produce　hydrogen　peroxide,　which　was　a　co-reactant　of　the　QDs.　The　electrochemiluminescence　signal　of　the　biosensor　increased　linearly　with　the　increase　in　the　Hg2+　concentration　over　the　range　from　1.0　x　10(-12)　to　1.0　x　10(-6)　M　with　a　detection　limit　of　1　x　10(-13)　M.　The　reproducibility,　stability　and　specificity　of　the　biosensor　was　also　studied.　The　biosensor　was　used　to　determine　Hg2+　in　a　real　water　sample　and　satisfactory　results　were　obtained.