The invention relates to a polyurethane waterproof mildew-resistant paint and a preparation method thereof. The paint comprises a polyurethane prepolymer, propyl hydroxyethyl methacrylate, dioctyl terephthalate, a titanate coupling agent NDZ-201, rutile titanium white, zinc oxide, a polysiloxane defoaming agent, talcum powder, a mixed solvent and a mildew preventive. The preparation method comprises the following steps: adding 1.7kg of 65-80 vol% ethanol into every kilo of propyl hydroxyethyl methacrylate, dissolving, mixing with the polyurethane prepolymer in a reaction kettle, reacting at 80-130 DEG C for 1.3-1.6 hours, and adding the titanate coupling agent NDZ-201; continuing the reaction for 4-6 hours; and adding the dioctyl terephthalate, rutile titanium white, zinc oxide, polysiloxane defoaming agent, mildew preventive, talcum powder and mixed solvent, and evenly mixing to obtain the polyurethane waterproof mildew-resistant paint product. The paint provided by the invention has favorable waterproof and mildew-resistant effects, and has the characteristics of high weather resistance and high pollution resistance.