Mercuric ion (Hg2+) is a hazardous pollutant widely exist in living organisms, foods and environments with highly toxic and bio-accumulative properties. In this work, a fluorescent probe based on aptamer gold nanoclusters (apt-AuNCs) was prepared for ultrasensitive detection of Hg2+ in food. The fluorescence of apt-AuNCs was quenched in the presence of Hg2+ due to the strong metallophilic interactions between the 5d10 centers of Hg2+ and Au+. Under optimal conditions, the proposed fluorescent probe displayed a linear relationship with Hg2+ concentration in the range of 2~ 200 nM (R2 =0.9960), the limit of detection (LOD) at 0.0158 nM, which is below the national standard value of 25 nM for Hg2+ in food. Furthermore, the apt-AuNCs can be applied for the determination of Hg2+ in real food samples, with recoveries of 91.99% ~ 108.06%.