The invention relates to a method for preparing a ferroferric oxide magnetic nanospheres. Soluble trivalent iron salt is taken as a raw material; in solution of glycol, urea is taken as a precipitating agent; and the ferroferric oxide magnetic nanospheres which have uniform particle sizes, high dispersibility in water and superparamagnetism are prepared through solvent thermal reaction. The preparation method comprises the following steps: preparing a trivalent iron salt mixed system; and then reacting and cleaning the product. The method solves the technical problems of poor dispersibility of the conventional ferroferric oxide magnetic nanospheres in the water, impure phases, impure product, wider particle size distribution and the like; and the particle size of the prepared nanospheres can be controlled according to reaction time as required. The method has the advantages of readily available raw materials, simple preparation process, and easy industrial production without complex and expensive devices.