To improve the selectivity of carbon dots to the photochemical recognition of mercury ions and the feasibility of its detection methods, a new kind of Nitrogen-Sulfur codoped carbon dots(NS-CDs)material with high fluorescence was prepared by thermal decomposition method and using citric acid and sulfanilamide as raw materials. Its structure and optical properties were characterized by infrared spectrometer, UV-Vis absorption spectrometer, transmission electron microscope, elemental analyzer, fluorescence spectrometer, etc. The results showed the quantum dot had graphite like structure, high water solubility and dispersibility, and its average particle size was about 4.78 nm. There are the following absorption peaks in the infrared spectra of NS-CDs: N—C and O—H bond vibrational absorption peaks at 3 446 and 3 261 cm-1, C—H bond vibrational absorption bands at 2 966 and 2 923 cm-1, the CC double bond vibrational absorption peak of the benzene ring skeleton at 1 630 and 1 570 cm-1, shear vibration peak of —CH3 at 1 388 cm-1, the vibrational absorption peaks of C—N, C—S, C—O, C—O—C and —SO-3 bonds at 1 268, 1 192, 1 146 and 1 071 cm-1, characteristic absorption peak of epoxy group at 912 cm-1 and the absorption band of N—H bond deformed vibration absorption band at 739 cm-1. It can be seen that the carbon dot contains not only the skeleton structure of benzene ring, but also the bonding structure in which N, S and other elements participate. An obvious and wide diffraction characteristic peak of (002) crystal plane appeared at 21.4° in NS-CDs. Its lattice distance (0.41 nm) was slightly larger than that of graphite (0.34 nm). The C, N, S and O element content of NS-CDs is 68.72%, 7.37%, 6.24% and 17.67%, respectively, which are consistent with the results of IR analysis. NS-CDs has a strong absorption peak at 309 nm, due to π→π* electron transition of CC bond, and a long tail in the visible region, and an absorption shoulder peak caused by the n→π* electron transition of the CO bond at 335 nm. When the excitation wavelength is less than 390 nm, the fluorescence emission peak of NS-CDs increases gradually with the increase of excitation wavelength, and the fluorescence intensity is the strongest at 390 nm, and weakens with the increase of excitation wavelength when the excitation wavelength is greater than 390 nm. At the same time, it is found that the emission peak gradually shifts red with the increase of excitation wavelength. When NS-CDs solution is diluted gradually, the optimal excitation peak shifts blue from 390 to 360 nm. When pH<11.0, the fluorescence intensity of NS-CDs varies very little, and it is the strongest at pH value 7.0. When pH>12.0, the fluorescence intensity of NS-CDs decreases sharply, so PBS buffer solution (pH7) was used to detect metal ions. Of the 16 metal ions, only Hg2+ has an extremely significant effect on the fluorescence intensity of NS-CDs, which completely quenches its fluorescence. Due to the high selectivity of NS-CDs to Hg2+ and the strong fluorescence quenching effect of NS-CDs by Hg2+, a new fluorescent chemical recognition method for Hg2+ by NS-CDs was established. The linear equation of the recognition method is y=5.559 02x-13.860 39, with a linear concentration range of 1×10-3~1×10-9 mol·L-1, R2 of 0.994 7 and the detection limit of 7.11×10-3 nmol·L-1. Its relative standard deviation is less than 9%, and it has high detection precision and recovery rate for practical samples, so it can be used for the detection of Hg2+ in real water samples with a good application prospect in the field of biological and environmental analysis.