Chlorides in the wastewater with high salt seriously interrupts the determination of chemical oxygen demand(COD). Therefore, chloride mask in wastewater has been an important and difficult issue in recent years. In the present work, experiment design is used for more accurate and precise COD determination method of pickled mustard wastewater. Two methods for masking chloride are adopted: HgSO4 masking method and AgNO3 masking method. After masking chloride, use the traditional reflux digestion method to test COD in the experimental sample. The results show that when the ratio of HgSO4 to Cl - is 40, the chloride mask results in a minimum error of 1.43%, and the minimum error is 0.21% when AgNO3 is used. Obviously, AgNO3 masking method is superior to HgSO4 masking method. However, when these two methods are used in pickled mustard wastewater, there is a COD value difference of 20 mg/L and HgSO4 masking method is more accurate. By considering the characteristics of the two methods above, a combined method is proposed, which AgNO3 is added equivalently according to the concentration of Cl - first, and then a few amounts of HgSO4 are added to prevent the free Cl - in wastewater. The facts show that combined method is more reliable.