The invention discloses a preparation method and applications of an aqueous polyurethane emulsion. The aqueous polyurethane emulsion is mainly composed of the following components in parts by weight: 50 to 100 parts of composite polyol with a carbonate structure and an ether structure, 0.05 to 0.2 part of catalyst, 10 to 80 parts of isocyanate, 1 to 12 parts of crosslinking agent, 2 to 10 parts of hydrophilic chain extender, 0 to 15 parts of non-hydrophilic chain extender, 30 to 100 parts of acetone, 1 to 12 parts of neutralizer, 150 to 400 parts of deionized water, 0 to 25 parts of filling materials, 0 to 30 parts of pigment, and 0 to 15 parts of auxiliary agents. The hard paint has the characteristics of low film-forming temperature (0 DEG C or more), quick drying, strong adhesive force, scratch resistance, and impact resistance, has a pencil hardness of 2H, can be used to paint materials such as metal, woodware, glass, plastic, leather, and the like, and has the advantages of wide application range, high performance-cost ratio, and vase market prospect.