The invention discloses a method for synthesizing (+ / -)-9-O-demethyl-alpha-dihydrotetrabenazine, which comprises the following steps of: adding sodium hydride and hexamethylphosphoramide into dimethylbenzene, heating to the temperature of between 90 and 100 DEG C, dripping N-methylaniline, and stirring for 15 to 20 minutes; dripping dimethylbenzene suspension of (+ / -)-alpha-dihydrotetrabenazine into the mixed solution, after dripping, continuously stirring the reaction suspension at the temperature of between 90 and 100 DEG C for 48 to 50 hours; slowly adding aqueous solution of sodium hydroxide into the reaction suspension, separating out an aqueous phase, neutralizing the aqueous phase by using aqueous solution of hydrochloric acid until the pH value is 7 to 9, extracting by using diethyl ether, concentrating an organic phase under reduced pressure to obtain a light brown solid; and purifying the light brown solid by using column chromatography technology to obtain the target product. By optimizing the reaction conditions of the prior art and improving posttreatment and purification technology, the method has the advantages of low preparation cost, high repeatability and high yield.