Deliberate addition of mildly oxidative chlorate (ClO₃⁻), so-called “chemical oxygen demand (COD) remover”, into wastewater in China or electrochemical production of ClO₃⁻ from Cl⁻ induces the false COD reduction, which would bring about false appearance of effluents meeting the COD discharge standards. In this study, an easy sulfite-based reduction method was developed for the first time to remove ClO₃⁻ from the water samples before COD determination to eliminate this interference of ClO₃⁻. In this reaction system, keeping the reaction temperature of sulfite reducing ClO₃⁻ at 60 ^°C was crucial for fast ClO₃⁻ removal rate, fixed molar [sulfite]_ini/[chlorate]_ini ratio value and the synchronous exhaustion of sulfite and ClO₃⁻, which were of great significance for the real application of this improved COD determination method. The ClO₃⁻ interference on COD determination could be successfully eliminated after 20 min reduction of ClO₃⁻ by sulfite at pH_ini 4.0∼6.0 with the molar [sulfite]_ini/[chlorate]_ini ratio value in the range of 5∼6 when concentration of ClO₃⁻ was below 5 mM. Despite of the involvement of SO₄^•− in the sulfite reducing ClO₃⁻ system, the degradation of organic matters by SO₄^•− could be greatly impeded due to the lessened dissolved oxygen for SO₄^•− production at high reaction temperature and the scavenging of SO₄^•− by sulfite. In this reaction system, ClO₂, ClO₂⁻ and ClO⁻ were also generated and could be further reduced by sulfite stoichiometrically via oxygen transfer process with Cl⁻ as the final product. In general, this study pioneered an effective, fast and convenient method for COD determination of the ClO₃⁻-laden wastewaters and evaluating the real electrochemical wastewater treatment performance in terms of COD removal.