<p>Fluorescent carbon quantum dots (CDs) have received widespread attention for their potential applications in optical sensing. Meanwhile, as the importance of mercury ion (Hg²⁺) detection in the environment, the exploration of Hg²⁺ fluorescent nanosensor based on CDs with high quantum yield is particularly intriguing. Herein, nitrogen-doped carbon quantum dots (N-CDs) were prepared by microwave method using citric acid as carbon source and urea as nitrogen source, and glycerol as microwave solvent. The N-CDs were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) technique, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The fluorescence quantum yield of the N-CDs is calculated to be as high as 60.51%. The N-CDs can specifically recognize Hg²⁺, and there exists a strong linear correlation between the concentration of Hg²⁺ and the fluorescence quenching intensity of N-CDs. The linear equation is <i>F</i>₀/<i>F</i> = 0.0062<i>c</i> + 1.5532, R² = 0.9936, and the detection limit is 0.080 µmol/L. The experimental results show that the recovery rate of Hg²⁺ in simulated wastewater ranges from 97 to 102%, and the relative standard deviation (RSD) is less than 3%, indicating that the prepared N-CDs can be used as a fluorescent probe for accurate detection of Hg²⁺.</p>