The invention discloses a preparation method of a dendritic waterborne polyurethane-based high-molecular dye, a polymer polyol and a dendritic polyester polyol are subjected to dehydration treatment for 2-3 hours under the conditions that the temperature is 100-110 DEG C and the vacuum degree is less than or equal to 0.1 mpa; under the protection of nitrogen, the dendritic polyester polyol, polyisocyanate and a catalyst are sequentially added into a reaction container for reacting for 3-8 hours at the temperature of 90-95 DEG C; the polymer polyol and a solvent are added for reacting at 80-90DEG C for 2-3 hours; a hydrophilic chain extender and a catalyst are added for reacting at 70-80 DEG C for 2-3 hours; a small molecular dye and a solvent are added for reacting for 2-3 hours at the temperature of 60-80 DEG C; after cooling to 30-50 DEG C, a neutralizer is added reacting for 30-60 minutes; a salt forming agent is added, and dispersing is performed for 20-30 minutes at a high speed;and in a rotary evaporator, rotary stirring is performed to remove the solvent at the temperature of 75-80 DEG C to obtain the dendritic waterborne polyurethane-based high-molecular dye the dye; andthe dendritic waterborne polyurethane-based high-molecular dye the dye has the characteristics of high temperature resistance, chemical resistance and corrosion resistance.