The invention discloses a high-efficiency decolorizing flocculant, particularly relates to a high-efficiency decolorizing flocculant for treating water-soluble dye-containing waste water and a preparation method thereof, belonging to the field of industrial waste water treatment. In the invention, a diamine or polyamine compound is added for reaction of dicyandiamide and formaldehyde to perform cross-linking condensation, so that the molecular weight is increased; and the obtained flocculant has high viscosity and high stability, has a better application effect when being used for waste watertreatment of dyes, printing and dyeing industry and the like, and overcomes the defects of low molecular weight, small floccules formed during actual industrial application, low sedimentation velocity, difficulty in post-treatment and the like existing in the conventional dicyandiamide and formaldehyde polycondensate flocculant.