The invention relates to a polyurethane emulsion which uses a prepolymerized monomer for one-time feed reaction and is double-bond termination. After the functional coating is coated, the double bond in the polyurethane molecular chains can participate in UV curing, at the same time, the three-component alcohol is used for intra-crosslinking to increase the degree of cross-linking of polyurethane molecular chains. The polyester film after high temperature transverse tensile curing is conducive to the penetration and spread of the functional layer of coating fluid. After UV aging for 72h, the adhesion between the functional coating and the polyester film can reach 100% and the functional coating and the polyester film do not fall off. The polyester film containing the undercoat layer provided by the polyurethane emulsion can be used as a diffusion film, a hard coating film and a brightness enhancement film.