In Lake Chenghai, annual increase in chemical oxygen demand (COD) has been observed since 2005, while biochemical oxygen demand (BOD) has remained almost constant and permanganate index(COD_Mn) has been increasing slowly. In order to reveal the reason for the phenomenon, typical inorganic anions such as Cl^-, F^-, S^2- and HCO₃^-, as well as dissolved organic matter (DOM) such as humic acid (HA), fulvic acid (FA) and commercial humic acid (SHA) in Lake Chenghai were surveyed for the effect of COD measurement. This study investigated the effects of inorganic anions and major DOM fractions, and their combined occurrences on COD measurement. Results showed that:Cl^- could significantly affect the COD measurement with the value of 5.42 mg/L, while the presence of S^2-, F^- and HCO₃^- may hardly impair accuracy of COD measurement; the inorganic anions also had a less impact on COD_Mn measurement. The different concentrations of HA, FA, SHA were significantly linearly correlated with COD values, the oxidation of 1 mg C HA, FA and SHA produced 2.164, 1.964 and 2.362 mg COD, respectively; while the oxidation of 1 mg C HA and FA produced 0.646 and 0.344 mg COD_Mn, respectively. The influence of DOM on COD measurement was significantly larger than that of COD_Mn; and the different concentrations of chloride ions enhanced the effect of FA and HA on COD determination. This study can build up the foundation for further exploring the internal causes for the increase in COD and COD_Mn in Lake Chenghai.