The invention provides a method for reducing a compound I. The method comprises: providing a sodium borohydride alkaline solution and a buffer solution; mixing a compound shown in formula I, an organic solvent and the buffer solution to obtain a mixture of which the pH is 7.0-8.0; adding the sodium borohydride alkaline solution into the mixture to reduce the compound shown in formula I, thus obtaining a reaction product containing a compound shown in formula II. By using the method, the compound of formula I can be effectively reduced; and the process is simple and convenient, and the reaction process is optimized, so that the method is very suitable for industrialized mass production.