The invention relates to a preparing method of ferroferric oxide nanometer particles with good dispersibility. The ferroferric oxide nanometer particles are prepared by steps of: adding ferric chloride and ferrous sulfate having the same molar as the ferric chloride into deionized water under the protection of an inert gas, so that the molar volume concentration of the ferrous sulfate containing ferrous iron and the molar volume concentration of the ferric chloride containing ferric iron are both maintained between 0.01-0.1 mol/L; adding a [BMIN]BF4 ionic liquid in place of a traditional surfactant; mixing uniformly; adding dropwise a sodium hydroxide solution adopted as a precipitator at 70 DEG C to precipitate to obtain a reaction product; washing alternatively with deionized water and absolute ethyl alcohol; centrifuging; and drying under vacuum. Raw materials used in the method are easily available, low in cost and short in preparation period. The obtained ferroferric oxide nanometer particles can be used as high-performance catalysts, magnetic materials and electrode materials.