The invention provides a nano waterproof breathable film, made from the following materials in percent by weight: 30-35% of low-density polyethylene, 15-20% of linear polyethylene, 12-18% of polylactic acid, 2-6% of hydroxyethyl cellulose, 11-16% of starch, 0.6-0.8 part of nano titanium dioxide, 10-15% of lecithin, 2-5% of tetraethyl orthosilicate, 3-8% of 2-hydroxysuccinic acid, 10-15% of polyvinyl alcohol, 2-10% of a plasticizer, and 2-10% of a lubricant. In connection with the preparation method provided herein, the waterproof breathable nano film can be produced; the film is imparted good water resistance and breathability; the produced nano film is light and thin, soft to touch, high in tensile strength and suitable for use as a surface material for sanitary products.