Nitrogen-doped carbon quantum dots (CDs) were synthesized in ethanol media by using citric acid (CA) as the carbon source and ethanediamine (EDA) as the nitrogen source. The obtained CDs were characterized by multiple analytical techniques including high resolution transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The prepared CDs can be constructed as a fluorescent probe for the detection of Hg²⁺ with good sensitivity and selectivity. Upon addition of I⁻, the fluorescence can be restored due to the strong interaction between Hg²⁺ and I⁻. The fluorescence can also be restored with the addition of Br⁻ by changing the buffer solution. The linear ranges and the limits of detection were 0–20 μM and 65 nM for I⁻, and 0–28 μM and 98 nM for Br⁻, respectively. In addition, the UV-vis spectrum of this sensing system was studied to validate the detection mechanism of Hg²⁺, I⁻ and Br⁻.