The invention discloses a method for preparing escitalopram represented by a formula (I), which comprises the following step: reacting S-diol represented by a formula (II) in an aprotic organic solvent under the action of a phosphine complex compound, an azo reagent and a proton supplying agent in an atmosphere of an inert gas to obtain the escitalopram represented by the formula (I). The method uses a Mitsunobu reaction in the cyclization preparation of escitalopram represented by the structural formula (I) for the first time and has the unexpected advantages of excellent product configuration retention, prevention of racemization in a cyclization process and high optical purity and yield. In addition, the method of the invention is simple in operation, mild in condition, simple and convenient in post processing and suitable for large-scale industrial production.