A fluorescent sensor was developed for detection of lead ion based on the replacement of double-stranded DNA and the difference of fluorescent intensity of the SG between single-stranded DNA and double-stranded DNA.As a kind of dye,SG could be inserted into the double-stranded DNA,making the SG fluorescence intensity stronger.The aptamer of lead ion reacted with its complementary strand to form a stable double chain.When lead ions existed in the solution,they could specifically bind with the aptamers to reduce the amount of double-stranded DNA.The lead ions could be quantificationally detected according to the fluorescence intensity when the SG was added into the solution.With the advantages of high sensitivity,strong specificity,simpleness and rapidness,the method has the lowest detection concentration of 2 nmol/L for lead ion,and a detection limit(S/N=3) of 1.6 nmol/L.