As a new advanced oxidation technology, advanced oxidation process of persulfate has been widely used in wastewater treatment and chemical oxygen demand(COD)is usually taken as an important indicator to identify the characteristics of organic wastewater. When COD is determined by potassium dichromate method, the residual persulfate in organic wastewater has interference with the measurement results. Therefore, the interference mechanism is investigated and the elimination measures of the interference are proposed. The results reveal that COD_Cr testing values are higher than the actual values for the reducibility of residual sulfate (S₂O₈^2-). The interference degree varies with the concentrations of S₂O₈^2- in the wastewater. The relationship between concentration of S₂O₈^2-and COD_Cr is obviously linear in both high and low COD_Cr concentration measuring range. For COD_Crranging from 0 to 150 mg/L and COD_Crranging from 0 to 1 500 mg/L, the linear relationship and the contributions of COD_Cr concentration variation by 1g S₂O₈^2- are y=36.120 2x+0.571 1(R²=0.997 9), 36.120 2 mg and y=45.171 6x+38.031 6(R²=0.998 4), 45.171 6 mg respectively. In COD_Cr standard solution system and the actual wastewater system, the percentage of △COD_Cr1 (contribution value of COD_Cr by S₂O₈^2-) and △COD_Cr2 (the difference between COD_Cr measured value and raw wastewater actual value) is less than 10% of the actual COD_Crin raw wastewater. The actual COD_Cr value of water sample could be accurately calculated by deducting △COD_Cr1 based on the linear equation which could effectively eliminate the interference caused by residual S₂O₈^2- in COD_Cr determination.