The invention discloses an intermediate for preparing diazepam-D5 and diazepam-D8 and a preparation method of the intermediate. The preparation method comprises the steps: reacting 6-chloro-2-methyl-4H-3,1-benzoxazin-4-one as a raw material with a deuterated bromobenzene Grignard reagent, and carrying out hydrolysis, acylation, cyclization, separation and purification to obtain the intermediate, namely 7-chloro-1,3-dihydro-5-(phenyl-d5)-2H-1,4-benzodiazepin-2-one, for preparing diazepam-D5 and diazepam-D8. The process disclosed by the invention has the characteristics of mild reaction condition, simplicity in operation and the like; and the deuterated diazepam intermediate prepared by using the preparation method is high in chemical purity, high in product quality and good in stability, can be used for preparing standard products of diazepam-D5 and can also be used for preparing standard products of diazepam-D8 and other relevant deuterated standard products.