The invention relates to a method for manufacturing nanometer ferric oxide, that is, a method for preparing ultrafine particles of ferric oxide, which belongs to the technical field of radiation chemistry manufacturing nanometer materials. A method for manufacturing nano ferric oxide, which is mainly prepared by radiation chemistry. In the process, ferric chloride is mainly used as the source of iron particles, ammonia water is used as a precipitant, and isopropanol (IPA) is used as an aqueous solution. Medium-oxidizing free radical scavenger; and control the growth rate of the crystal nucleus to control the particle size through the hydrophilic surfactant polyvinyl alcohol (PVA), and use the electron beam generated by the electron accelerator to irradiate, and the radiation The irradiated solution is washed, centrifuged and dried to obtain black nanometer ferric oxide powder. The invention has simple technological process, short production cycle, no pollution, no pollution and good safety. The prepared nano ferroferric oxide has extremely small magnetic saturation strength and coercive force, and is a very good supermagnetic material and soft magnetic material.