The invention relates to a preparation method of pharmaceutical intermediates, in particular to a preparation method of o-aminosulfonyl-benzoyl hydrazine, belonging to the intermediates preparation field of organic synthesis. The preparation method comprises the following steps of: (1), firstly, determining that the feed ratio of o-sulfonylbenzoylimine to hydrazine hydrate is 1mol to 1-10mol; at the room temperature, adding the o-sulfonylbenzoylimine to a container, adding anhydrous alcohol as a solvent, and stirring to fully mix; and under the ice-bath condition of 5 DEG C below zero-5 DEG C, dropwise adding the hydrazine hydrate while stirring; (2), heating the solution to back flow within 30 minutes when the solution changes clear; and continuously reacting for 1-15 hours at the backflow temperature; and (3), transferring the reaction liquid to a beaker for naturally cooling to the room temperature, standing to separate out granulous white crystals, washing with water and alcohol, and drying to obtain the final product, i.e. o-aminosulfonyl-benzoyl hydrazine. In the invention, the preparation method has the advantages of simple operation, easy control, one-step finished reaction, high purity, innocuity and the like.