The invention belongs to the field of chemical engineering, and relates to a preparation method of 4-bromo phthalic anhydride. The preparation method includes the following steps that sodium hydroxide is dissolved in water, phthalic anhydride and a phase transfer catalyst are added and evenly mixed, a reaction is performed in the mode that programmed temperature control is performed in three stages and reactants are added in three times, 20% fuming sulphuric acid is added for acidification, a sodium bisulfite aqueous solution is added after cooling for removing excessive bromine, an organic solvent is added for extraction, the organic solvent is removed through distillation, then temperature is increased to obtain a crude 4-bromo phthalic anhydride product, rectification dehydration is performed under the vacuum condition, fraction is collected, and the finished 4-bromo phthalic anhydride product is obtained. Compared with a multi-step reaction in the prior art, the process steps are simplified, the product yield is high, the production energy consumption is reduced, the three-waste emission is greatly reduced, the energy consumption and wastewater treatment cost for extraction of 4-bromophthalic acid crystals are reduced, production cost is reduced, and environmental protection pressure is relieved.