In current COD measurement method, we found that sodium chlorate (NaClO3) can effectively reduce the chemical oxygen demand (COD) of minerals processing wastewater (MPW). However, the reduction in COD via the present COD method was only achieved through the interference of NaClO3 with COD measurement. A series of laboratory-scale experiments demonstrated that the COD reduction is independent of NaClO3 reaction time, reaction pH and reaction temperature, but only related to NaClO3 dosage, and a higher NaClO3 dosage leads to a lower COD. Results further showed that the addition of NaClO3 didn't affect the initial concentration of the total organic carbon, chloride ions and the benzohydroxamic acid (BHA); their concentration remained unchanged after the addition of NaClO3. Moreover, the residual concentration of NaClO3 also remained same as the initial value. However, COD in both MPW and BHA solution decreased. Furthermore, when added reducing agent to consume the residual NaClO3 before COD measurement, and then the COD was determined to return to its initial value. All these observations clearly shown that NaClO3 doesn't react with the reducing substances in wastewater under the experimental conditions; it only reacts with reducing substances under the COD measurement conditions. From all these evidences, we concluded that NaClO3 can interfere with the determination of COD. In fact, the interference of NaClO3 is not specified in the standard method, which is a defect of the present COD measurement method. Residual NaClO3 remains in wastewater may cause secondary pollution to the environment. This study, therefore, suggest the government revise and improve the COD measurement method in order to avoid the interference of NaClO3 and prevent the industrial use of NaClO3 for COD reduction of wastewater.