The invention relates to a method for preparing Ritonavir applied to the technical field of pharmaceutical synthesis. According to the method, under the conditions of organic alkali and an organic solvent, HATU is used as a condensing agent, N-[2-Isopropylthiazol-4-ylmethyl(methyl)carbamoyl]-L-valine and (2S,3S,5S)-5-Amino-2-(N-((5-thiazolyl)-methoxycarbonyl)amino)-1,6-diphenyl-3-hydroxyhexane are subjected to an acidamide reaction, and then a certain post-treatment is performed, so that the Ritonavir is obtained. According to the method disclosed by the invention, the synthesis yield of the Ritonavir is increased, the purity is high, and the cost of raw materials is effectively reduced. Besides, the reaction time is short, the feed is simple, nitrogen protection is not needed, and feeding temperature needs to be appropriately controlled, so that by-products of HATU can be easy to wash away, the preparation time is greatly shortened, the working efficiency is improved, and the method is suitable for industrial production.