Compared with traditional quantum dots, carbon quantum dots (CQDs) have aroused tremendous research interest among scientists as new fluorescent carbon nanomaterials due to their unique properties (easy surface functionalization, good biocompatibility, and low toxicity). The nitrogen-doped carbon quantum dots (NCDs) was synthesized with one-step hydrothermal treatment with citric acid as carbon source, and ammonia as nitrogen source. The obtained NCDs exhibited excellent water-solubility and high stability. High-resolution Transmission electron microscopy (HRTEM) image showed that the diameter of particles was about 3nm. Fourier transform infrared (FIIR) and X-ray photoelectron spectroscopy (XPS) spectra analysis demonstrated that the NCDs are functionalized with hydroxyl, amino, carbonyl, and carboxylic acid groups, which indicated NCDs has better water-solubility. It was further demonstrated that such NCDs can serve as effective fluorescent sensing platform for Hg2+ ions detection with sensitivity and selectivity. In a PBS butter solution (0.1 mol·L-1 pH 7.0), the fluorescence quenching ratio (F/F0) indicated linear responses with Hg2+ concentration ranging from 0.001 to 0.1 μmol·L-1, with the detection limit of 2.1 nmol·L-1. The fluorescence technique was superior in terms of sensitivity, selectivity and simplicity. It was successfully applied to determination of Hg2+ in samples.