The invention discloses a fly ash washing process for multistage reverse pulping. The fly ash washing process comprises the following steps: 1, mixing fly ash and water according to a solid-to-liquid ratio of 1: (3-5), and pulping; 2, performing suction filtration on the pulping product in the step 1 to obtain water washing liquid and solid residues; 3, mixing the solid residues obtained in the previous step with water according to a solid-to-liquid ratio of 1: (3-5) to prepare slurry; 4, performing suction filtration on the pulping product in the step 3 to obtain water washing liquid and solid residues, and mixing the water washing liquid with the water washing liquid obtained in the step 2; 5,sequentially repeating the step 3 and the step 4 for multiple times, and obtaining mixed washing liquid and solid residues after repeating is completed; 6, mixing the solid residues obtained in the step 5 with water according to a solid-to-liquid ratio of 1: (5-8) to prepare slurry; and 7, performing suction filtration on the pulping product in the step 6 to obtain water washing liquid and solid residues, concentrating the water washing liquid by using a DTRO membrane to obtain concentrated water and fresh water, recycling the concentrated water in the step 1 to be mixed with the fly ash for pulping, and recycling the fresh water in the step 6 to be mixed with the solid residues for pulping.