Citric acid and ethylene-diamine were used to synthesize nitrogen-doped carbon dots by hydrothermal reaction. The fluorescence changes of carbon dots under different pH conditions were systematically studied. The results showed that the fluorescence of the carbon dots gradually decreased,and the rate of decline was faster in the solution of pH 9.0 than in the solutions of pH 5.0 and pH 7.4 as the illumination time increased. On this basis,UV-visible absorption spectroscopy and Raman spectroscopy were used to investigate the fluorescence quenching mechanism of carbon dots after illumination. It was found that the illumination destroyed the conjugated π-electron structure of the carbon dots,so that the fluorescence was quenched,and the dissolved oxygen in the solution had no significant effect on the photostability of the carbon dots.