The invention discloses deuterated diazepam and a preparation method thereof. The preparation method of deuterated diazepam comprises the following steps of: (1) mixing 7-chlorine-1-methyl-5-phenyl-1,3-dihydro-1,4-benzodiazepine-2-ketone and N,N-dimethyl formamide, and stirring; (2) adding potassium carbonate and deuterated chloroform, heating the mixture to more than 40 DEG C and stirring; and (3) carrying out separation to obtain the deuterated diazepam. The preparation method provided by the invention is short, simple and convenient to operate, low in cost and easy to purify. The commercially available non-deuterated diazepam uses a small amount of deuterated reagents as deuterium sources in a non-deuterated solvent atmosphere, so that deuterated diazepam is obtained in a relatively short period of time, and through simple column chromatography purification, a purified product is purified. The deuterated diazepam standard product prepared according to the invention is high in purity and stable in chemical property; and the preparation of the standard product for analysis is convenient. The preparation method provided by the invention can be used for producing deuterated internal standard substance used for analyzing and detecting diazepam.