The invention discloses a method for preparing nanometer high-active-oxygen stabilized sodium percarbonate at the normal temperature. The method comprises the following steps of adding phosphate intoa saturated sodium carbonate solution, uniformly stirring, and putting in a storage tank for standby use; adding a stabilizing agent into a crystallizer filled with hydrogen peroxide, starting to stir, and dissolving; under the stirring condition, adding a solution in the storage tank into the crystallizer; after dropwise adding the total solution in the storage tank, adjusting the stirring speed,continuously stirring, decompressing, filtering, and drying. The sodium percarbonate prepared through the method has higher roundness, and shows a spherical shape through a microscope; meanwhile, through tests, the active oxygen content of the product reaches up to 13 percent or above, and the active oxygen content is at least 12.85 percent when the product is stored for 35d. The preparation method provided by the invention has the advantages of simple process, low energy consumption, simple equipment, low investment and low cost.