A method for purifying (S)-oxiracetam is disclosed. Crude (S)-oxiracetam with the purity of 89% is dissolved in water; the solution is allowed to stand for 1 to 3 days at 0° C. to 18° C.; a colorless transparent crystal is precipitated; the solution is filtered and top-washed with cold water to 0° C. to 5° C.; and the product is dried in vacuum to obtain high-purity (S)-oxiracetam.