The invention discloses a method for producing sludge modifier by utilizing coal gangue, comprising the following steps: fully and evenly mixing the coal gangue and carbonate, and calcining; soaking the calcined solid matter by water, and filtering to obtain filter residue and alkaline filter liquor; dissolving the filter residue by adopting the acid liquor, and filtering to obtain the alkaline filter liquor; mixing the acidic filter liquor and the alkaline filter liquor; respectively adjusting the alkalinity and the acidity of the mixing system and the concentration of SiO3<3->, Al<3+> and Fe<3+>; carrying out polymerization reaction; carrying out spray drying after the polymerization reaction to obtain the sludge modifier taking polysilicate aluminum iron as a main component. Compared with the prior art, the technical scheme of the invention activates the coal gangue by utilizing a high temperature condition, simultaneously realizes the extraction and separation of Si element, simplifies the production process, and adopts the low-price carbonate as an auxiliary raw material, thus saving the production cost, and multi-element extraction is carried out on the raw materials of the coal gangue, thus improving the use ratio of the raw materials of the coal gangue.