The invention discloses a method for synthesizing tetrabenazine. The method comprises: step one, using a dimethylamine aqueous solution and a formaldehyde aqueous solution as initial materials to be reacted1 to obtain tetramethyl methane diamine; step two, dissolving the tetramethyl methane diamine obtained in the step one in an organic solvent, dropwise adding acetyl chloride and 5- methyl-2-hexanone, and performing amine methylation to obtain an intermediate 3-[(dimethyl amino) methyl]-5-methyl-2-hexanone; and step three, reacting the 3-[(dimethyl amino) methyl]-5-methyl-2-hexanone obtained in the step two with 6,7-dimethoxy-3,4-dihydroisoquinoline hydrochloride to obtain the tetrabenazine. According to the method for synthesizing tetrabenazine, the cheap and accessible materials are used as the initial materials, imine salts serve as amine methylation reagents to perform amine methylation reaction on the 5- methyl-2-hexanone, and accordingly, the region-selectivity of chemical reactions is improved; and water serves as a reaction solvent to prepare the tetrabenazine, so that the operation is simple and convenient, no complex post-processing process exists, and good industrial application prospects are provided.