NOVELTY - A biodiesel is prepared by mixing 30-50 g ferrous sulfate heptahydrate and ferric sulfate hydrate with mass ratio of 1:2 with 250-500 ml distilled water, slowly adding 20% concentrated aqueous ammonia until pH is greater than or equal to 8, stirring to obtain black precipitate, decanting supernatant to obtain magnetic fluid, filtering, washing, drying at 50-60 degrees C for 8-10 hours to obtain magnetic substrate, dipping in magnesium acetate solution with equal volume, drying at 50-60 degrees C for 8-10 hours, and calcining at 500-600 degrees C for 2-4 hours to obtain magnetic carrier magnesium oxide/ferrous ferric oxide.    USE - Preparation of biodiesel (claimed).    ADVANTAGE - Biodiesel has high efficiency. Process easily separates catalyst and product, and is simple.    DETAILED DESCRIPTION - A biodiesel is prepared by mixing 30-50 g ferrous sulfate heptahydrate (FeSO4.7H2O) and ferric sulfate hydrate (Fe2(SO4)3.xH2O) with mass ratio of 1:2 with 250-500 ml distilled water, slowly adding 20% concentrated aqueous ammonia until pH is greater than or equal to 8, stirring to obtain black precipitate, decanting supernatant to obtain magnetic fluid, filtering, washing, drying at 50-60 degrees C for 8-10 hours to obtain magnetic substrate, dipping in magnesium acetate (Mg(Ac)2) solution with equal volume, drying at 50-60 degrees C for 8-10 hours, calcining at 500-600 degrees C for 2-4 hours to obtain magnetic carrier magnesium oxide/ferrous ferric oxide (MgO/Fe3O4), dipping in calcium acetate (Ca(Ac)2) solution with equal volume, calcining for 500-600 degrees C for 2 hours to obtain calcium oxide/magnesium oxide/ferrous ferric chloride (CaO/MgO/Fe3O4), placing peanut oil, methanol, and catalyst into 500 ml 3-necked flask, placing into super thermostat to control time and temperature, standing to divide into layers in which upper yellow transparent liquid is biodiesel, middle yellow transparent liquid is glycerol, and lower gray solid is catalyst, washing catalyst residue with petroleum ether, and absorbing catalyst by using magnet.