Levorotatory optically active compound (S-configuration) of prulifloxacin for treating infection represented by the following formula (1) and its preparation method are disclosed. The said method uses levorotatory ulifloxacin as the starting material and carries out the reaction in an organic solvent and in the presence of an alkaline material. The reaction temperature is -20℃~60℃ and the reaction time is 15 minutes to 24 hours.