The invention discloses a synthetic method of 3, 5-dichloro-2, 4-difluoroaniline. The synthetic method takes distillation residues which are generated in the manufacturing technology that 2,6-dichlor fluorbenzene is nitrified to prepare 2, 4-dichloro-3-fluoronitrobenzene as raw material, and the distillation residues are chlorinated through chlorine gas and fluorated through potassium fluoride so as to prepare 3, 5-dichloro-2, 4-difluoro nitrobenzene; and finally, 3, 5-dichloro-2, 4-difluoro nitrobenzene is reduced to obtain 3, 5-dichloro-2, 4-difluoroaniline. The synthetic method adopts by-products that are generated in the conventional manufacturing technology line as raw material, is directly used for synthesizing 3, 5-dichloro-2, 4-difluoroaniline without any treatment, is characterized by low price of raw material, full resource utilization, mild reaction conditions, high yield coefficient of products, low manufacturing cost and the like compared with a conventional method for preparing 3,5-dichloro-2,4-difluoroaniline, is easy to get raw material, and is very suitable for industrialized production.