The invention relates to a preparation method of an osimertinib intermediate, comprising: subjecting ortho-nitrotoluene and 3,3-dialkoxypropanenitrile as raw materials to alkali-catalyzed nucleophilicaddition to obtain 1-(2-nitro)phenyl-4,4-dialkoxy-2-butanone, subjecting 1-(2-nitro)phenyl-4,4-dialkoxy-2-butanone and N,N-dimethylformamide dimethyl acetal to thermal condensation to obtain 1-dimethylamino-2-(2-nitro)phenyl-5,5-dialkoxy-3-n-pentanone, subjecting the attained reaction liquid to direct catalytic hydrogenation to obtain 3-(3,3-dialkoxy)propionylindole, subjecting 3-(3,3-dialkoxy)propionylindole to reaction with a methylation reagent under alkaline conditions to generate 3-(3,3-dialkoxy)propionyl-N-methylindole, and subjecting 3-(3,3-dialkoxy)propionyl-N-methylindole and 2-methoxy-4-fluoro-5-nitrophenylguanidine to exocondensation to obtain the osimertinib intermediate. The materials herein are low in price and easy to attain, the route is short, and the preparation method is environmentally friendly and high in yield.