Abstract: The 2 alkaline digestion methods (sodium oxalate alkali method and potassium iodide alkali method) were used to determine the potassium permanganate index (COD_Mn) in water Body, and the potassium iodide alkali method was optimized. 100 mL of sample was taken and placed in a 250 mL-cone shaped bottle. 0.5 mL of 500 g·L⁻¹ sodium hydroxide solution and 10.00 mL of 0.01 mol·L⁻¹ potassium permanganate standard solution were added. The mixture was shaken well, and heated in boiling water bath for (30±2) min, then quickly cooled. 5 mL of 25% (volume fraction) sulfuric acid solution and 0.5 g of potassium iodide were added immediately. The mixture was quickly shaken, and avoided light for 5 min. The solution was titrated with 0.01 mol·L⁻¹ sodium thiosulfate standard solution until the color turned light yellow. 1 mL of 5 g·L⁻¹ starch solution was added and the solution was continue titrated with sodium thiosulfate standard solution until the blue or dark brown color disappeared. The volume of consumed sodium thiosulfate standard solution was recorded. The differences in the determination of COD_Mn in glucose, glycerol, resorcinol, and actual samples by potassium iodide alkali method and sodium oxalate alkali method in the presence of chloride ions were compared, and the influence mechanism of chloride ions was analyzed by combined the reaction process at different stages. As shown by the results, when the content and types of organic compounds in the sample were different, chloride ions could affect the determined value of COD_Mn. It was difficult to obtain a unified interference threshold of chloride ion for actual samples. The potassium iodide alkali method had better resistance to chloride ion interference compared to the sodium oxalate alkali method.