The invention relates to a preparation method of (S)(-)-amisulpride. According to the preparation method, by introducing an R2 group which is selected from benzyl, p-methoxybenzyl, 2,4-dimethoxybenzyl, p-nitrobenzyl and p-methylbenzyl, on one hand, the steric hindrance is increased, the side reaction is avoided, on the other hand, amino is activated, the nucleophilicity of amino is increased, and the condensation reaction can be easily carried out at mild conditions. The preparation method has the characteristics that the raw materials are low in toxicity and easily available, and the reaction conditions are mild; the generation of chiral isomer dextroisomers is avoided; the preparation method is applicable to the industrial production.