The invention relates to the field of medicine synthesis, particularly to a preparation method for reducing a lactone structure into hemiacetal, particularly to a preparation method for a darunavir intermediate. According to the method, a compound shown in a formula I is reduced under the conditions of alkali, aluminum trichloride, a reducing agent and a solvent to prepare a compound shown in a formula II, or the compound shown in the formula I is reduced under the conditions of alcohol, alkali, aluminum trichloride, a reducing agent and a solvent to prepare the compound shown in the formula II, wherein R1, R2, R3, R4, R5 and R7 are hydrogen, hydroxyl, amino, alkyl, aryl, fluorine, chlorine or bromine, R6 and R8 are hydrogen, hydroxyl, amino, benzoyloxy, alkyl, aryl, fluorine, chlorine orbromine, and n is 0 or 1.