The invention relates to a method for reducing excess sludge of an aerobic unit of a sewage treatment system by an activated sludge process, which comprises a step of adding a chemical uncoupling agent into water inlet of the aerobic unit of the sewage treatment system, wherein the chemical uncoupling agent comprises the following components: 3,3,4,5-tetrachloro salicylamide, soluble copper salt and an adsorbent; the soluble copper salt is one of copper sulfate, copper chloride or copper acetate; the adsorbent is one of refined diatomite or zeolite powder or a mixture of the refined diatomite and the zeolite powder; the weight ratio of the 3,3,4,5-tetrachloro salicylamide to the soluble copper salt is 1:0.05-0.25; the weight ratio of the total of the 3,3,4,5-tetrachloro salicylamide and the soluble copper to the adsorbent is 1:0.05-1.00; and in the water inlet, the addition amount of the chemical uncoupling agent accounted by the 3,3,4,5-tetrachloro salicylamide is controlled in the range of 0.6 to 3.8mg / l. The method has stronger sludge reduction performance. If the same weight reduction rate of the sludge is achieved, the added TCS (tetrachloro salicylamide) can be reduced by 25 to 75 percent, and the removing rate of the COD (chemical oxygen demand) is basically not influenced.