The invention relates to a preparation method of (S)-oxiracetam, which comprises the following steps: reacting raw materials glycine ethyl ester hydrochloride and ethyl (S)-4-halo-hydroxy-butyrate in an alcohol solvent under alkaline conditions, filtering, washing with inorganic alcohol, concentrating, extracting, concentrating the water phase, introducing stronger ammonia water to react to obtain a (S)-oxiracetam crude product, and carrying out purification treatment on the crude product. The glycine ethyl ester hydrochloride is firstly mixed with the alkali and alcohol solvent, and the raw material ethyl (S)-4-halo-hydroxy-butyrate is dropwisely added and the alkali is added in different batches to control the pH value of the reaction at 8-9. The purification treatment comprises the following steps: dissolving the (S)-oxiracetam crude product in a benign solvent to prepare a saturated solution at room temperature, and dispersing with a poor solvent in a closed environment. The HPLC (high performance liquid chromatography) purity of the prepared (S)-oxiracetam is up to 99.0% %, and the yield can be up to 33%; and the method is mild in reaction conditions, simple to operate and beneficial to industrial large-scale production.