The invention relates to a preparation method of ferroferric oxide nanoparticles, relating to the field of preparation of inorganic non-metallic nano materials. The preparation method comprises the steps of: firstly, preparing a mixed dispersion solution of a ferric source and a ferrous source and a dispersion solution of an alkali source, adding the prepared mixed dispersion solution of the ferric source and the ferrous source into the dispersion solution of the alkali source, stirring for reaction for 50-70min to generate black precipitates in a reaction process, delivering reactants to a centrifuger, washing the separated precipitates sequentially with distilled water, a water-alcohol solution and absolute ethyl alcohol, centrifuging, separating and drying to obtain ferroferric oxide nanoparticles. In the invention, a used reaction solvent has the advantages of low cost and no toxicity and harmfulness, can be recycled and has no environment pollution problem; and used equipment is simple, operation conditions are easy to master, and thus, large-scale production is convenient.