Pyrite, usually associated with gold deposit, is disadvantageous to gold leaching. In this study, an environmentally friendly oxidizing agent, chlorine dioxide (ClO2), was used for the first time to efficiently oxidize pyrite under mild experimental conditions, so as to remove the adverse effects of pyrite on gold leaching. Firstly, with established content determination method, the prepared pure ClO2 solutions with different concentrations were investigated on their impacts on pyrite oxidation efficiency. Secondly, solid NaClO2 was used to oxidize pyrite under acidic condition, providing a new oxidation approach for practical application. In fact, the key of the approach is the use of solid NaClO2 in acidic environment to generate ClO2 and then oxidize pyrite. For the pyrite pulp with 400 mL volume with an initial concentration of 0.025 M, the oxidation efficiencies of Fe and Sin Pyrite reached 99.8% and 98% at a NaClO2 concentration of 0.125 M and C4H6O6 concentration of 0.0025 M with magnetic stirring for 1 h at 30 degrees C in a constant temperature water bath. The addition of C4H6O6 can increase the acidity of the system and form a complex with iron ions, thereby preventing the precipitation of ferrihydrite from acidity changes caused by overdose NaClO2. After redox reaction, the acidity and oxidation-reduction potential (ORP, E-h) of the system were noticeably increased. The results of chemical composition analysis and X ray diffraction (XRD) analysis of pyrite oxidation residue confirmed that pyrite was oxidized to ions by ClO2, and no solid by-products such as Fe2O3 or Fe(OH)(3) were formed. The scanning electron microscope (SEM) analysis of residual pyrite indicated that small pyrite particles were oxidized first, and then oxidants eroded the defects of large particles, gradually oxidizing all pyrites. This method is expected to be used as a pretreatment approach for pyrite oxidation before gold leaching process.