The invention relates to a preparing method of disperse blue dye. The preparing method includes the following steps that 1), 2,4-binitro-6-chloroaniline is added into a sulfuric acid solution containing sodium nitrite to carry out diazo reaction so as to obtain diazonium liquid; 2), water and solvent are added into a coupling reactor to obtain reaction base solution, and the mass fraction of solvent in the reaction base solution is 10-20%; 3), 2-methoxy-5-acetamido-(diethyl) aniline is added into the reaction base solution, pulping is carried out when the pH value is adjusted to be 2-4, the temperature is reduced to minus 5-0 DEG C, the diazonium liquid with the volume amount accounting for 30-60% is added for coupling reaction, water continues to be added, remaining diazonium liquid is added to obtain reaction solution, coupling reaction continues to be carried out until coupling is finished, and the disperse blue dye is obtained through filtering and cleaning; the mass fraction of the solvent in the reaction solution is 1-10%. The disperse blue dye prepared through the preparing method is fine in crystal, high-temperature dispersity of the dye is improved, conventional commercialization is achieved, and the product requirement is met.