In　this　work,　we　reported　a　simple　coumarin-boronic　acid　ester　fluorescent　probe　1,　which　displayed　very　high　＂ON-OFF＂　fluorescence　response　for　Hg2+　in　an　aqueous　environment　(H2O/CH3CN　=　1:1,　v/v)　or　in　a　PBS　solution　(pH　=　7.4).　With　the　addition　of　Hg2+　to　the　solution　of　probe,　a　color　change　from　light　yellow　to　orange　could　be　observed　by　naked　eye.　The　detection　limits　were　lower　than　the　permitted　level　of　Hg2+　in　drinking　water　defined　by　WHO.　Infrared　spectroscopy,　mass　spectroscopy　and　H-1　NMR　titration　studies　showed　that　probe　1　formed　a　1:1　complex　with　Hg2+.　In　addition,　the　probe　was　non-toxic　to　cells　and　could　stain　specifically　the　membrane　region　surrounding　the　HeLa　cell.