The invention belongs to the technical field of dyes, and particularly relates to a preparation method of a cationic blue dye. According to the invention, a compound II is used as a diazonium component and a compound I is used as a coupling component for synthesizing the cationic blue dye for the first time; and the method has the advantages of simple process, environmental friendliness, high product purity, high yield and the like. The results of multiple production experiments with scales not below pilot tests show that mother liquor (such as mother liquor in steps 2, 3 and 4) in the reaction process can be cyclically used 6-8 times without treatment, so the amount of high-salt, high-COD and high-chroma wastewater is greatly reduced, a working environment is effectively improved while product yield is further increased and reaction reagent cost is saved, and the product still keeps high purity; and thus, product quality is guaranteed, production cost is reduced, and obvious economicbenefits, social benefits and environmental benefits are produced.