The selective chlorination reaction of the aromatic ring among the aromatic compounds is very important in the economy and environmental protection aspects. A novel chlorination reaction system HCl-H₂O ₂-CH₃COOH was applied to react with o-xylene. Compared to the traditional Lewis acid catalysts which comprised sulfur and zeolite catalytic chlorination system, it took on much superiority such as high-selectivity, high-yield, simple operation, savings and environmental protection etc. The reaction conditions were controlled, the molar ratios of hydrogen peroxide, hydrochloric acid to o-xylene were 5.0,4.0 respectively, the reaction was processing under 70 ℃ until the feeding was finished and then the reaction was 2 h under 90 ℃. The conversion ratio of the raw material is over 99%, the yield is 96.9% and the selectivity is over 99% under this condition.