Abstract: In order to ascertain the occurrence characteristics and sources of organic matter in the sediments of Nanhu Lake water system in Jiaxing, the continuous extraction and three-dimensional fluorescence spectroscopy were used to systematically analyze the content, composition, source and stability of the organic matter in surface sediments of the lake. The results showed that the total organic matter content in surface sediments ranged from 5.96 to 40.04 g/kg, with an average value of 15.69 g/kg. Average content of water-soluble organic matter (WSOM), fulvic acid (FA), Humin (HM) and humic acid (HA) were 0.62, 5.22, 5.00, 4.85 g/kg, respectively. Four fluorescent components were identified in the surface sediment WSOM, including two humus-like components (C2 and C3) and two protein-like components (C1 and C4). Among the four components, C1 was dominated by tryptophan-like fluorescence peaks, C2 by fulvic acid fluorescence peaks , C3 by humic acid fluorescence, and C4 by tyrosine fluorescence. The average value of C/N ratio in surface sediments of Nanhu Lake water system was 7.71, and the fluorescence index (FI) and humification index (HIX) were 1.54-1.78 and 1.62-7.76, respectively, indicating that the source of organic matter in the surface sediments of Nanhu Lake water system was endogenous, which accounted for a large proportion, and mainly from the processes of metabolism of its own microorganisms, and the degree of humification was weak. The correlation analysis showed that the fluorescent components of WSOM in surface sediments were significantly positively correlated with both ammonia nitrogen (AN) and water-soluble organic nitrogen (SON) (P<0.01), which showed high bioavailability.