In this study,a “turn off” fluorescent sensor based on aptamer functionalized grapheme oxide(GO) decorated gold nanoparticles(AuNPs) composites was developed for the detection of Hg2+.Firstly,the GO@AuNPs with certain fluorescence properties were fabricated by wet chemical method,then the sulfhydryl modified single chain T-rich nucleic acid DNA(aptamer) was immobilized on the surface of gold nanoparticles to form aptamer functionalized graphene oxide decorated gold nanoparticles complexes(aptamer-GO@AuNPs).In the presence of Hg2+,due to the coordination of seven T-Hg2+-T structures,T-rich aptamer was folded to form a rigid and double stranded hairpin structure,making coordinated Hg2+ close to the graphene surface(less than 1 nm).In that case,electrons could transfer from graphene to mercury ion along the double stranded DNA channel,which caused the graphene fluorescence quenched.Based on this,a “turn-off” fluorescence sensor was constructed.Under the optimal experimental conditions,the linear detection range for Hg2+ was 0.5-80 nmol/L,and the detection limit was 0.3 nmol/L.The method was applied in the detection of Hg2+ in environmental water samples.The recoveries ranged from 96.0% to 105% with the relative standard deviations of 1.4%-3.2%.With the advantages of simple operation,strong anti-interference,high sensitivity and selectivity,label-free and rapid detection,the method could be used for the high sensitive detection of Hg2+ in environmental water samples.