A　one-step　reduction　method　was　used　for　the　preparation　of　stable　graphitic　carbon　nitride-gold　nanoparticles　(g-C3N4-Au)　nanocomposites　from　ultrathin　g-C3N4　nanosheets　and　chloroauric　acid　by　using　NaBH4　as　a　reducing　agent　under　ultrasonication.　The　nanocomposites　were　characterized　by　transmission　electron　microscopy,　X-ray　photoelectron　spectroscopy,　UV-Vis　absorption　and　fluorescence　spectroscopy　etc.　The　results　revealed　that　the　gold　nanoparticles　(AuNPs)　are　uniformly　formed　on　the　g-C3N4　nanosheets.　It　is　found　that　the　peroxidase-like　catalytic　activity　of　this　nanocomposite　for　the　oxidation　of　3,3,5,5-tetramethylbenzidine　by　H2O2　to　form　a　blue-colored　product　is　strongly　enhanced　in　the　presence　of　Hg(II).　Based　on　this　phenomenon,　a　sensitive　turn-on　colorimetric　assay　for　Hg(II)　was　developed　that　works　at　physiological　pH　values.　Under　optimal　conditions,　the　absorption　signal　at　652nm　increases　linearly　with　Hg(II)　concentration　in　the　range　from　5　to　500nM.　A　detection　limit　as　low　as　3.0nM　was　achieved.　This　assay　has　excellent　selectivity　over　other　metal　ions.　It　was　successfully　applied　to　the　determination　of　Hg(II)　in　real　water　samples.　The　method　is　cost-effective,　rapid,　and　allows　for　visual　detection.