The invention discloses a nonionic aqueous polyurethane emulsion and a preparation method thereof, belonging to the technical field of high-molecular materials. The nonionic aqueous polyurethane emulsion comprises polyether polyol or polyester polyol, isocyanate, micromolecular chain extender, nonionic hydrophilic chain extender, hydrophilic chain extender, 0.6-1 weight portions of cross-linking agent, lower boiling point solvent, catalyst, amine neutralizer, amine chain extender, 0.4-2 weight portions of aqueous wetting agent, and aqueous levelling agent. The preparation method is characterized in that: prepolymerization, polymerization, emulsification, distillation, compounding are carried out to prepare the nonionic aqueous polyurethane emulsion, then the aqueous wetting agent and aqueous levelling agent are added with stirring to obtain an aqueous nonionic polyurethane glass fibre impregnating compound. The prepared nonionic aqueous polyurethane emulsion disclosed herein has the advantages of good stability, long stable phase, wide application scope, high strength, wear resistance, good elasticity, yellow stain resistance, aging resistance, solvent resistance, low temperature resistance, tortuosity resistance, good penetration, compatibility, good interface chemical combination or chemisorption and other excellent performances.