The invention relates to a waterborne polyurethane external extinction finishing agent and a preparation method thereof. The waterborne polyurethane external extinction finishing agent is prepared from the following components of polyester polyhydric alcohol, diisocyanate, extinction powder, a hydrophilic chain extender and a secondary amine type chain extender, wherein the polyester polyhydric alcohol adopts hexanedioic acid poly-neopentyl glycol with side groups, the diisocyanate adopts aromatic isocyanate, the hydrophilic chain extender adopts dyhydroxy carboxylic acid, and a solvent contains a strong-polarity non-protonation solvent; the contents of the hexanedioic acid poly-neopentyl glycol and aromatic isocyanate is 25% to 35%, the content of the extinction powder is 0.5% to 0.7%, the content of the strong-polarity non-protonation solvent is 20% to 50%, the amount of the secondary amine type chain extender accounts for 20% to 30% of the amount of the aromatic isocyanate, the content of the dyhydroxy carboxylic acid is 5.0% to 7.0%, and the content of antioxidant is 0.6%. The preparation method adopts the polyurethane prepolymer dispersing method. The waterborne polyurethane external extinction finishing agent has the advantages that the cost is low, the finishing effect, water washing-resistant property, low temperature-resistant and adhesion force are good, the preparation technology is simple, and the like.