The invention discloses a method for preparing a decoloring flocculant of reactive dye waste water. The method comprises the following steps of: firstly, adding a proper quantity of water into a reactor, and then adding dicyandiamide, aluminum sulfate and one half of a formaldehyde solution with a concentration of 37 percent, wherein the adding molar ratio of the dicyandiamide to the formaldehyde with the concentration of 37 percent to the octadeca-hydrate aluminum sulfate is 1:(1.5-3.0):(0.2-0.5); heating the reactor as well as keeping stirring, stopping heating when the temperature reaches 40-45 DEG C and reacting for 0.5-1h, wherein the temperature in the reactor can slowly rise; adding the other half of the formaldehyde solution with a concentration of 37 percent when the temperature does not rise, gradually heating up to 80-90 DEG C, reacting for 2-3h and then stopping heating; and finally, laying up and cooling. The prepared decoloring flocculant has good removal effect on reactive dye waste water difficult to treat in printing and dyeing waste water, and moreover, the synthesis cost of a dicyandiamide-formaldehyde decoloring agent is also reduced, thus the method has the advantage of high flocculation speed, is less affected by coexisting salts, the pH and the temperature, and the like.