The invention discloses a preparation method of lamivudine and an intermediate thereof; (2R,5R)-5-hydroxyl-[1,3]oxathiolane-2-carboxylicacid(2S-isopropyl-5R-methyl-1R-cyclohexyl)ester, the structure of which is shown in formula (II) is used as a raw material; acyl compound, the structure of which is shown in formula (III) is obtained by the acylation reaction of acyl; acyl compound (III) reacts with cytimidine, the structure of which is shown in formula (IV) by condensation reaction to prepare the intermediate of lamivudine (V) and the intermediate is reducted to obtain lamivudine (I). The invention is characterized by safe and reliable operation, simplified process, low production cost and the like, thus being suitable for industrialized production.