Abstract: Strengthening the water environmental governance in Nanhu Lake and its surrounding waters is significant for improving the water quality in the Hangzhou-Jiaxing water net. Using ultraviolet-visible absorption and three-dimensional fluorescence spectroscopy, the distribution of DOM, fluorescence characteristics, and the correlation between components and major water quality parameters were analyzed in different water stages of Nanhu Lake water system in 2018-2019, which offered further insight into the components, resources, and bioavailability of DOM in Nanhu Lake water system. Experimental results showed that DOM concentration of Nanhu Lake water system in dry season (5.63 mg/L) was significantly higher than that in wet season (4.81 mg/L). 3 protein-like peaks and 1 humus-like peak were resolved from three-dimensional fluorescence spectrum of DOM in Nanhu Lake water system, and it was found that the contribution rate of protein-like peaks to the total fluorescence intensity was 90.80%, and the contribution rate in dry season was higher than that in wet season. The distribution of DOM and its components showed spatial differences, with a trend of high in the north and west and low in the southeast, while the fluorescence index showed temporal trend of being higher in dry season (1.75) than in wet season (1.70). Both humus index and biogenic index showed that the bioavailability of organic matter in Nanhu Lake water system was high, and it was higher in dry season than in wet season. There was a significant positive correlation between DOM, ammonia nitrogen and organic nitrogen. DOM in the water body of Nanhu Lake mainly came from urban domestic source discharge and internal source release, and there existed sewage direct discharge or pipe network leakage in the northwest region. Controlling the exogenous DOM input can effectively control the nitrogen level in the water system.