The invention discloses a preparation method of a fly ash-based treatment agent for oily sewage. The method comprises the following steps: performing acid treatment on fly ash, performing alkali treatment, and performing modification by using a mixed modifier (a mass ratio of glyceryl monooleate to sodium hypochlorite to sodium dichloroisocyanurate of 2:1:1) to obtain the fly ash-based modifier. According to the method provided by the invention, the current solid waste fly ash is used as a raw material, modification treatment is performed on the fly ash to enrich the pore structure, and dopingmodification is performed on the fly ash through a specific component and a specific process to obtain the treatment agent with excellent performance for the oily sewage. The treatment agent providedby the invention has good application prospects.