The invention relates to a method for preparing vat brilliant red RB (Algol Scarlet B), comprising the following steps: adding 3, 4, 9, 10-perylene tetracid dianhydride into KOH water solution and heating the solution to fully dissolve it, and cooling the solution to 50 degrees centigrade and neutralizing KOH in the solution by H2SO4 until the pH value equals to 5-6, and orange perylene tetra-formic acid is separated out; filtering and washing the perylene tetra-formic acid, and mixing a perylene tetra-formic acid filter cake with water, and adding parachloroaniline and pyridine into the solution, and uniformly stirring them, and transferring the material into a pressure kettle and closing the kettle, and raising the temperature to 130-150 degrees centigrade and performing heat preservation reaction until the perylene tetracid dianhydride is used up, wherein the reaction pressure is 0.2-0.4 MPa; cooling the material to the room temperature, and filtering it and washing it by hot water until no green fluorescence to obtain a vat brilliant red RB rough product filter cake, and heating and processing the rough product filter cake by sodium hypochlorite solution, and filtering, water washing and drying the cake to obtain a vat red RB dry product. The invention provides a safe, environment-friendly and economic preparation method.