The invention provides a method for degrading azo dye by cooperatively activating PMS through Cl<-> and a CNT.The method comprises the steps that a certain amount of sodium chloride (NaCl), the carbon nano tube (CNT) and peroxymonosulfate (PMS) are added into a water body containing the azo dye (OG), the pH of a system is regulated to be and keeps within 2-10, reacting under continuous stirring is performed for 45 min at a certain revolving speed and certain temperature, and the azo dye (OG) is removed through oxidative degradation.According to the method, two mechanisms are related, that are, Cl<-> and PMS are subjected to a non-free-radical reaction to generate HOCl to oxidatively degrade the azo dye, the CNT activates PMS to generate a free radical reaction, a sulfate radical SO4<-> is generated to oxidatively degrade the azo dye, and the two mechanisms jointly act to achieve efficient degradation on the azo dye; the azo dye can be quickly and efficiently oxidatively degraded at normal temperature, the suitable pH range is wide, and the method is very suitable for treatment of industrial dye wastewater with the high salt content.