Hydrogen peroxide interference on chemical oxygen demand (COD) measurement has been a big problem in the application of the Fenton process. However, there is no simple and effective method available to address this problem, although several methods have been reported in the literature. In this study, a new method has been developed based on Na2SO3 reduction and O2 oxidation, which has easy operation and short time requirement. Na2SO3 reduction was used to remove H2O2 in water samples, which was independent of pH in the investigated range of 2.50–11.95. Residual Na2SO3 was removed by subsequent O2 oxidation, and effects of initial solution pH, ferric ion dosage, and stirring speed were explored. Solution pH below 3.0 and stirring speed of 700 rev min−1 could ensure a sufficiently high oxidation rate for Na2SO3 with ferric ion higher than 0.469 mM. This new method was proven to be effective in the matrix of Fenton treating real landfill leachate. Meanwhile, the procedure for this method in other applications was proposed in detail. To the best of our knowledge, this newly developed method is the most simple and effective way to avoid H2O2 interference on COD analysis.