The invention discloses a method for preparing lisinopril intermediate (S)-2-((S)-2,5-dioxo-4-(4-(2,2,2-trifluoroacetamido)butyl) oxazolidine-3-yl)-4-phenylbutyrate. The method comprises the followingsteps: adding an organic base, III (i.e. L-proline), a reaction solvent to a reaction vessel under the protection of nitrogen, adjusting the temperature to 10-45 DEG C, adding the solution of I, keeping the temperature, stirring, and concentrating the reaction solution under reduced pressure to obtain the product IV. The invention provides an anhydrous condensation preparation method, which can control the hydride residue to be less than 0.5%, and the subsequent refining process can be purified and removed to ensure the product quality.