The invention relates to a preparation method of a red complex dye. The preparation method comprises the following steps: uniformly mixing dissolving liquid of a compound (V) with a water solution ofa compound (VI), controlling the temperature, regulating the pH value, and reacting by virtue of the compound (V) and the compound (VI) under the effect of a dispersing agent, so as to obtain a compound (IV); preparing a water solution with a mass ratio of 10%-20% from the compound (IV), reacting by virtue of the water solution and a complexing agent at a pH value of 3.5-7.0 and a temperature of 100-120 DEG C, reacting at a pH value of 1.0-2.0 and a temperature of 120-140 DEG C, and filtering, so as to obtain a compound (III); uniformly mixing diazonium salt of a compound (VII) with a water solution of a compound (VIII), regulating the pH value at 6.5-10.5 and the temperature at 5-30 DEG C, and reacting for 6-10 hours, so as to obtain a compound (II); and uniformly mixing the compound (III) with the compound (II) in water, regulating the temperature at 65-95 DEG C and the pH value at 6.0-9.5, completely reacting, cooling to obtain particles, and carrying out spray drying, so as to obtain a compound (I). The preparation method has the beneficial effects that the production process is simplified, the production cost is lowered, the method is relatively economic and environment-friendly, and the obtained product has relatively excellent and stable performance.