The present invention relates to a process for preparing 4-chloro-3-hydroxybutanoic acid ester, an intermediate for preparing atorvastatin, in high purity and yield, by comprising the steps of 1) reacting epichlorohydrin of formula (2) with cyanide of formula (3) under the condition of pH ranging from 7 to 8, to form the 4-chloro-3-hydroxybutyronitrile of formula (4) and 2a) dissolving the 4-chloro-3-hydroxybutyronitrile of formula (4) in an alcoholic solvent and reacting it with hydrogen chloride, or 2b) reacting the 4-chloro-3-hydroxybutyronitrile of formula (4) in an alcoholic solvent saturated with hydrogen chloride, to form the 4-chloro-3-hydroxybutyronitrile acid ester of formula (I).