The invention relates to a method for synthesizing a medicine intermediate, that is, a method for preparing (2S,3R)-2-amino-3-hydroxy-3-(4-methylsulfonyl phenyl) ethyl propionate (I). The method comprises the following steps: putting a compound (2S,3R)-2-amino-3-hydroxy-3-(4-methylsulfonyl phenyl) ethyl propionate (II) into an organic solvent, performing a heating reaction with aldehyde and a resolving agent, cooling, filtering, and further performing alkali freeing in the solvent, thereby obtaining a target compound of formula I. The method provided by the invention is high in product yield, simple and convenient and beneficial to industrial production.