(−)-(S)-3-(3′-hydroxy)-butylphthalide (a compound shown by Formula I) and an ester formed of the same and an acid are proved by experiments to be applicable to treatment and prevention of cerebral ischemic diseases and have a sleep-improving function. The acid refers to a pharmaceutically acceptable inorganic or organic acid. The inorganic acid refers to nitric acid, sulfuric acid, or phosphoric acid. In addition to an acid radical, the organic acid at least comprises at least one of an amino group, a hydroxyl group, and a carboxyl group. None of the compound shown by Formula I and the ester thereof is water-soluble. An ester generated from the compound and the acid further react with an acid or a base to generate a salt which is water-soluble and is used to prepare injection preparation. The experiment proves that the salt does not stimulate vessels.