The invention provides a method for treating surplus sludge produced after biological treatment of wastewater. By adding a ferrous sulfate solution, a sodium persulfate solution or a potassium persulfate solution, and industrial coal ash, the sludge is treated to obtain a filter cake and a filter liquor. Compared with the prior art, adding divalent ferric salt and heating synergize to stimulate the persulfate to produce a free sulfate radical having a strong oxidizing property, the free sulfate radical is used to efficiently break a microbial cell in the sludge, then the microorganism is inactivated, the adhesive action with an extracellular polymeric substance is remarkably weakened, the extracellular polymeric substance is then stripped, the bond-state water is released, and the structure and the surface hydrophobicity of the sludge flocculant are changed; after the industrial coal ash is added, the permeability of the filter cake is improved, and the filter cake can play the role of a filter aid. According to the method, the specific resistance of the sludge is as low as (0.2-0.5)*108S<2>/g, the moisture content of the dehydrated filter cake is as low as 40-60%, the secondary pollution caused by common flocculants like polyacrylamide is reduced, the operating cost for the dehydration of the sludge is lowered, original processing equipment can be used to implement the method without any transformation, the operation is convenient, and the dehydrated sludge is directly backfilled or is used to produce a ceramsite or building material.