The invention relates to a method for synthesizing diethyl 2-[2-(2,4-difluorophenyl)allyl]-1,3-malonate. The method comprises the following steps: dropwisely adding 1,2,3-trichloropropane into 1,3-difluorobenzene, adding aluminum trichloride, extracting and washing; drying with anhydrous Na2SO4, filtering, evaporating to obtain 1,3-dichloro-2-(2,4-difluorophenyl)propane, adding the 1,3-dichloro-2-(2,4-difluorophenyl)propane and potassium hydroxide into an alcohol, and refluxing; removing the alcohol, adding ice water, neutralizing with hydrochloric acid at 5 to -5 DEG C, extracting with dichloromethane three times, drying with anhydrous Na2SO4, filtering, adding the obtained 1-chloro-2-(2,4-difluorophenyl)-3-propanol and potassium hydrogen sulfate into chlorobenzene, and heating under reflux; washing with water to a neutral state, drying with anhydrous Na2SO4, filtering to obtain 1-(1-chloromethylvinyl)-2,4-difluorobenzene, dissolving the 1-(1-chloromethylvinyl)-2,4-difluorobenzene in DMSO (dimethyl sulfoxide), adding diethyl malonate and hydroxide to react, extracting, washing, and carrying out reduced pressure distillation to obtain the target product. The synthesis route is disclosed in the specification.