A simple and rapid colorimetric method was developed for the detection of Hg2+ based on the specific thymine T -Hg2+ -T structure and gold nanoparticles.A tweezers like duplex DNA，containing 9 T-T mismatches in the end，was designed.The mismatched thymines in the tweezers like dsDNA could not form helix structure,which could attach to AuNPs and effectively prevent AuNPs from salt induced aggregation.While in the presence of Hg2+ ions，the T-Hg2+-T coordinations would lead these thymines to form the structures of helix.The AuNPs were less well-protected thus aggregated at the same salt concentration，accompanied by color changing from red to blue.The maximum absorption wavelength of AuNPs solution would change from 520 nm to 620 nm.Under the optimal conditions，the absorption ratio A620/A520 was proportional to Hg2+ concentration in the range of 5.0×10-8-5.0×10-7 mol/L with a detection limit of 3.0×10-8 mol/L.The interferences from selected inorganic ions were investigated.The result indicated that the sensor showed a good selectivity for Hg2+，and other metal ions （e.g. Ca2+，Mg2+） had no significant interferences.The proposed method was simple and rapid，and could be used in the detection of Hg2+ within 1 min.