The invention discloses a garbage incineration fly ash filter cake modification method including: first, adding fly ash, sodium carbonate, coal ash and water into a first-stage water washing stirring tank for water washing for three times, using a vacuum filter for solid-liquid separation of filtrate and filter cake after each time of water washing; wherein filtrate separated by first time solid-liquid separation is sent into a sewage tank for subsequent wastewater treatment; the second time and third time solid-liquid separated filtrates are respectively used as the first time and second time water washing water; the third time water washing is clear water washing. The third time solid-liquid separated filter cake is a modified fly ash filter cake, due to the less looseness and smaller viscosity, the feed conveying is smooth, drying machine material attachment is reduced, the strength of material blocks formed during drying of the filter cake is small, wall scrapping is easy, and scattering is smooth, the fly ash filter cake drying process can be performed normally, the problems existed in the real engineering can be solved, and the cement kiln co processing garbage incineration fly ash technology can be improved.