The invention relates to a polyurethane modified polycarboxylate superplasticizer and a preparation method thereof. The preparation method comprises the following steps: pouring a component A into polyurethane prepolymer at normal temperature, dropwise adding components B and C, and cooling to room temperature after dropwise addition is finished, adjusting the pH value to obtain the polycarboxylate superplasticizer, wherein the component A is prepared by mixing and stirring polyoxyethylene ether, polyethylene glycol and water and then adding a tin or acid catalyst and hydrogen peroxide; the component B is obtained by mixing acrylic acid, a reducing agent and water, and the component C is obtained by mixing sodium methallyl sulfonate, an initiator and water. The water reducer provided by the invention has the branch structure having a plurality of arms and long enough arm lengths, the steric hindrance effect can be brought into full play, the water reducing performance is excellent and the concrete working performance is improved; the method disclosed by the invention is mild in reaction condition, simple in process, less in side reactions, safe, environment-friendly, and low in energy-consuming and production cost.