The invention provides water-repellency packaging paper based on nanometer plant fibers and a preparing method thereof. The preparing method includes the following specific steps that rice straw is dried in the sun and beaten, the product is soaked in a sodium hydroxide solution, heated, boiled and activated, the pH value is adjusted to be neutral, and pretreated rice straw is obtained; the pretreated rice straw is put into a biological enzyme solution, the mixture is subjected to ultrasonic stirring, inorganic acid is dropwise added, the mixture is subjected to ultrasonic stirring, the product is taken out and filtered with an ultra-filtration membrane, washing is carried out, a hydrogen peroxide solution is dropwise added for bleaching, and a solution containing the nanometer plant fibers is obtained; fillers, cationic starch, bentonite and gummy-substance chemical medicine are added into the solution containing the nanometer plant fibers, the mixture is stirred to be even to form pulp, the pulp is distributed on a forming net, dehydration and drying are carried out, and the water-repellency packaging paper based on the nanometer plant fibers is obtained. According to the water-repellency packaging paper prepared with the method, the rice straw serves as a raw material, nano-cellulose whiskers are formed with the biological chemistry and physical technologies and then prepared into the packaging paper, and the packaging paper has the water-repellency effect accordingly.