Intake of water contaminated with mercury ions leads to various toxic effects and health issues. In this work, a novel gold nanoparticles (AuNPs) DNA hybrid sensor for fluorescent detection of mercury(II) ions (He2+) in aqueous solution was prepared. The fluorescein was modified onto the terminal of ssDNA and it consequently presented a strong fluorescence. The probing behaviors toward various metal ions were investigated through the fluorescence intensity change. The result demonstrated that the amplification strategy based on ssDNA-AuNPs for Hg2+ detection by using DNase I exhibited high sensitivity and selectivity. Under optimum condition, the resultant chemosensor tested the concentration of mercury ions in a linear range from 10 to 300 nM (R-2 = 0.9637), and a detection limit as low as 2.11 nM. Moreover, the developed sensor was used to detect Hg2+ ions in the real samples, and the observed results were in good coordination with the value of calculation.