Coprecipitation–oxidation method was developed to recover the iron from wastewater. Fe O nanoparticles were well synthesized from steel waste pickling liquor. Promoters greatly improved the properties of synthesized Fe O nanoparticle. Real-time control of the Fe /Fe molar ratio was achieved by ORP monitoring. Waste pickling liquors (WPLs) containing high concentrations of iron and acid are hazardous waste products from the steel pickling processes. A novel combined coprecipitation–oxidation method for iron recovery by Fe O nanoparticle production from the WPLs was developed in this study. An oxidation–reduction potential monitoring method was developed for real-time control of the Fe /Fe molar ratio. The key coprecipitation–oxidation parameters were determined using the orthogonal experimental design method. The use of promoters greatly improved the Fe O nanoparticle crystallinity, size, magnetization, and dispersion. X-ray diffraction patterns showed that the produced Fe O nanoparticles were single phase. The Fe O nanoparticles were approximately spherical and slightly agglomerated. Vibrating sample magnetometry showed that the Fe O nanoparticles produced from the WPLs had good magnetic properties, with a saturation magnetization of 80.206 emu·g and a remanence of 10.500 emu·g . The results show that this novel coprecipitation–oxidation method has great potential for recycling iron in WPLs.