The invention relates to a self-cleaning size for yarns, a self-cleaning yarn and a preparation method thereof. The self-cleaning size for yarns is prepared from the following components in parts by weight: 40 to 60 parts of starch, 25 to 45 parts of polyvinyl alcohol, 1 to 6 parts of biological enzyme, 1 to 10 parts of nano-nickel oxide, 1 to 8 parts of Na-montmorillonite, 1 to 8 parts of metal naphthenate, 3 to 15 parts of organic alcohol, 1 to 8 parts of coupling agent, 1 to 8 parts of cross-linking agent, 1 to 8 parts of dispersant and 100 to 150 parts of water. The self-cleaning size for yarns disclosed by the invention can ensure that highly complex multiple nanoscale and micron-scale ultramicrostructures are formed on the surface of a yarn, consequently, a lotus effect is generated, and the yarn obtains a self-cleaning characteristic like a lotus leaf; moreover, under the effect of light, the self-cleaning size for yarns can generate a strong catalytic degradation function, consequently, odor can be effectively removed, staining can be effectively resisted, a variety of bacteria and fungi can be killed, and the yarn cannot be easily stained, and is easy to clean. The size has good adhesion to the yarn, the polyvinyl alcohol is highly compatible to the starch, furthermore, the flexibility of a size film can be enhanced, and the mechanical property of the yarn can be increased.