The invention belongs to the field of drug synthesis, and particularly relates to a preparation method of a tolvaptan intermediate 7-chloro-2,3,4,5,10,11-hexahydro-1H-1-benzodiazepines-5-one. The method comprises the following steps: by taking N-vinylpyrrolidone as an active ingredient, rearranging under light to obtain 4-azecycloheptatrienone, then protecting nitrogen methyl p-toluenesulfinate, reacting with 2-chloro-2-butenal in the presence of diaryl D-prolinol derivatives to generate 7-chloro-2,3,4,5,10,11-hexahydro-1H-1-benzodiazepines-5-one, dehydrogenizing and aromatizing in the presence of manganese dioxide, and finally removing protection groups to obtain the 7-chloro-2,3,4,5,10,11-hexahydro-1H-1-benzodiazepines-5-one. Due to no Claisen condensation or Friedle-Crafts reaction in the conventional synthesis method, the potassium tert-butanol and aluminum trichloride are omitted, the preparation method is mild in reaction conditions, easily available in raw materials, capable of lowering the cost, and more suitable for industrial production.