In this paper, three-dimensional fluorescence spectroscopy was used in order to find out the three-dimensional fluorescence characteristics of DOM components during the treatment of brackish water by the “ultrafiltration-nanofiltration” dual-membrane water treatment process. Raw water mainly contains two high-intensity spikes and two low-intensity spikes. With the progress of the water treatment process, the fluorescence peaks in water samples gradually weaken. Quantitative analysis was performed on the water samples in the process of water purification by parallel factor analysis. It was found that the water sample mainly contains protein-like and fulvic-like factors. The variation of DOM components was analyzed by the maximum fluorescence peak intensity of each factor in the water sample. The fluorescence peak intensities of protein-like factors and fulvic-like factors in raw water were 0.351 08 and 0.175 55, respectively. The fluorescence intensity of the protein-like fluorescence peak is greater than that of the fulvic acid-like fluorescence, which indicates that the raw water has been contaminated by external organic matter. After the conventional process, the removal of the protein-like factors and the fulvic acid-like factors was 22.27% and 47.57%, respectively. The DOM that is not completely removed in the conventional process will enter the “ultrafiltration-nanofiltration” dual-membrane water treatment process. The ultrafiltration unit does not have a good removal effect on the DOM, which is a pretreatment to ensure that the water quality could meet the requirements of the nanofiltration unit. The removal of DOM is mainly contributed by nanofiltration unit. After the nanofiltration treatment, the DOM in the water is greatly reduced. The fluorescence peak intensities of the two factors were 0.013 67 and 0.002 56, respectively. Compared with the raw water, the protein-like substances have decreased 96.11%, fulvic-like substances decreased by 98.54%. Therefore, in treating brackish water by dual-membrane process, the proper pre-oxidation treatment should be designed to enhance the removal of DOM to reduce the impact on membrane fouling. This paper will provide a further theoretical basis for the promotion of the dual-membrane process and the control of membrane fouling.