The invention discloses a preparation method of a waterborne polyurethane emulsion and the waterborne polyurethane emulsion prepared by the preparation method. The preparation method comprises the following steps: (1) synthesizing a polyurethane prepolymer: mixing and dehydrating polypropylene oxide polyether glycol and polycarbonate diol, dropwise adding toluene diisocynate after dehydrating for heat-preserving reaction, and finally dropwise adding prepared mixed solution of dihydroxybutyric acid, 1, 4-butanediol and acetone for heat-preserving reaction to finally prepare the prepolymer; (2) adding aminoalkyl silicon oil, an inner crosslinking agent and hydroxy-propyl acrylate to modify; (3) neutralizing to salify: dropwise adding an acetone solution of a neutralizer triethylamine into the modified prepolymer prepared in the step (2) for reaction so as to prepare a polyurethane salified solution; and (4) dissolving disulfamate and a diamido chain extender in water, quickly adding to the polyurethane salified solution which is stirred at a high speed, quickly dispersing and extending chains, and heating the emulsion to remove the solvent through evaporation to obtain the waterborne polyurethane emulsion.