Chemical oxygen demand (COD) is an important index used to assess organic oxygen consumption pollution. To explore COD composition in the natural water in Baiyangdian Lake, the main composition, source, and influencing factors of oxygen-consuming organic substances in the water body were revealed through physical continuous classification, three-dimensional fluorescence, and other methods. The results showed that the COD of the two waters was affected by dissolved organic substances (protein-like and humus-like organic matters) with size of less than 220 nm (59%-93%), and inorganic substances had little effect on COD. The source of organic matter in overlying water was primarily affected by endophytic vegetation decomposition, sediment release (the release flux of TOC was in the range of 1.55-2.28 mg·(m2·d)-1), and other endogenous sources (biological index>0.8), as well as by land-based sources such as reed platform and artificial pollution (1.4