The invention discloses a preparation method of dibenzenesulfonimide. The preparation method comprises the following steps: (1) preparing dibenzenesulfonimide sodium salt: adding benzene sulfonamide and benzene sulfonyl chloride into an isopropanol-sodium alcoholate system in different steps to carry out reactions; (2) purifying dibenzenesulfonimide sodium salt: adding sodium hydroxide into the isopropanol-sodium alcoholate system in different steps, adjusting the pH value to 10-12, making the system go through anion exchange resin, then cooling to precipitate crystals, and carrying out centrifugal separation to obtain dibenzenesulfonimide sodium salt; (3) preparing dibenzenesulfonimide: dissolving dibenzenesulfonimide sodium salt into hot water, making the solution go through cation exchange resin, cooling to precipitate crystals, carrying out centrifugal separation, and drying to obtain dibenzenesulfonimide. The preparation method has the advantages that raw materials are fully and effectively converted into the target product, the side reactions are avoided, the purification and separation mode is reasonable, the product purity is high, the yield is high, and moreover, the method is economic and environment-friendly and can be easily applied to industrial production.