A DNA sequence was designed as a specific probe for Hg2+ and a novel label-free fluorescent aptasensor was constructed with the help of the minor groove binding dye DAPI.In the absence of Hg2+，DNA folds into a stable stem-loop structure and DAPI inserts into the stem portion causing strong fluorescence emission,and in the presence of Hg2+,the configuration of DNA produces change and DAPI could release,which results in the decrease of fluorescence.The experiment results showed that the percentage of fluorescence decrease is proportionally correlated with concentration of Hg2+ at the optimal experimental conditions,and a good linear relationship was obtained even in the low concentration of 0.5-50 nmol/L.The actual detection limit is 0.5 nmol/L.With the advantages of short detection time and simple operation,the method has a potential application for the rapid real-time detection of Hg2+ in environmental water samples.