The invention discloses a preparation method of sulfadimidine hydrosulfate. The preparation method comprises the following steps: at first, preparing a mixed sulfuric acid solution, then adding sodium hydrogen sulfite/water solution and sulfadimidine into the mixed sulfuric acid solution, dissolving and then filtering, thereafter crystallizing, filtering, washing and filtering, and finally drying in vacuum to obtain sulfadimidine hydrosulfate. The preparation method disclosed by the invention is simple to operate, suitable for large scale production and convenient for recycling and reuse of a solvent due to use of a single solvent, and simultaneously lowers the production cost, and improves the production safety, yield and product quality. Sulfadimidine hydrosulfate prepared by the preparation method has high yield, good quality, low solvent residue and good stability.