The invention provides a preparation method of bromine ammonia blue. The preparation method comprises following steps: carrying out a condensation reaction between bromamine acid and 2,4,6-trimethyl-3,5-diaminobenzenesulfonic acid with a catalyst system containing a cuprous complex for catalyzing the reaction and then performing seperation to obtain the product bromine ammonia blue (sodium 1-amino-4-((3-amino-2,4,6-trimethyl-5-sulfonylphenyl)amino)-anthraquinone-2-sulfonate) after the reaction finished. By means of utilization of the catalyst system containing the cuprous complex, usage amounts of cuprous chloride and copper powder can be effectively reduced, thereby reducing the content of heavy metals in a dye product, reducing the content of heavy metals in waste water and reducing environmental pollution and production cost. In addition, the preparation method can avoid generation of a hydrolyzed by-product (a purple by-product) of the bromamine acid, can greatly reduce the generation of a debromination by-product, can significantly reduce the generation of a double-condensation substance, can increase yields of the condensation reaction and the dye product and can reduce discharge amount of waste water containing acids and being high in chroma.