Diazepam keto acid drug intermediates benzoic anhydride synthesis method, comprising the following steps: equipped with a stirrer, a thermometer, a fractionating filled column, the reaction vessel was added 12.6mol benzoic acid (2) solution, 300ml acetonitrile solution, 13.5-13.8mol benzamide solution (3), 0.56mol aluminium oxide, the solution temperature was raise to 110--115 C, reacted for 3-5h, the temperature of the solution is reduced to 80-85 C, the reaction was continued to 90-120min, the temperature of the solution is reduced to 15-20'C, the precipitated solid was washed with salt solution, washed with dimethylamine solution, dehydrated with dehydrating agent, vacuum distillation, collecting fractions of 130--135 0C , recrystallized from cyclohexane solution, got crystals of benzoic anhydride.
