The invention discloses a fipronil self-nanoemulsion solution, and a preparation method and an application thereof. The fipronil self-nanoemulsion solution is mainly prepared from, by weight, 1-20 parts of fipronil, 20-50 parts of an oily solvent, 40-80 parts of an emulsifier and 10-40 parts of a co-emulsifier. The oil-in-water fipronil nanoemulsion has the advantages of uniformity in distribution, good transparence and stability, low surface tension, good fluidity, convenience in use, narrow particle size distribution, small particle size, increase of the absorption rate of the effective component, and good in vitro permeability. The preparation method has the advantages of simplicity, convenience, and easiness in production and application; and the emulsion can be directly used in in-vitro parasite expelling of pets as a spray or a drop, and can be diluted with water to form the oil-in-water nanoemulsion having a small particle size, a good stability and a high pet skin permeabilityin order to expel in vitro parasites from the pets, and the above preparation is clinically used for treating the parasites of the pets.