NOVELTY - H-type resin and iron(III) ion in methanol solution or iron(III) ethanol solution are placed in flask, and stirred to obtain modified resin. The modified resin is washed, and dried to obtain ferric ion-modified strongly acidic styrene cation exchange resin heterogeneous catalyst. The heterogeneous catalyst and food waste oil are added to flask, and short-chain alcohol is added to obtain mixed solution. The mixed solution is heated to obtain a mixture containing biodiesel and glycerol byproduct. The obtained mixture is separated, distilled, dried and clarified to obtain biodiesel.    USE - Preparation of biodiesel using modified resin catalyst (claimed).    ADVANTAGE - The method provides biodiesel without using base as catalyst, by simple and environmentally-friendly process, without equipment corrosion. The biodiesel prevents deacidification preprocess. The catalyst has high catalytic activity, and excellent reusability and stability, and is prepared by a simple and economical process.    DETAILED DESCRIPTION - 732-Sodium-type-strong acidic styrene cation exchange resin is converted to H-type resin using sodium hydroxide solution and hydrochloric acid solution having preset concentration, and washed with distilled water. H-type resin is mixed with anhydrous methanol or ethanol to make the resin swollen, and dried. The dried H-type resin and iron(III) ion in methanol solution or iron(III) ethanol solution are placed in 100 ml flask, and stirred at predetermined temperature to obtain modified resin. The modified resin is washed with anhydrous methanol or ethanol till the resin is colorless, and dried at room temperature to obtain ferric ion-modified strongly acidic styrene cation exchange resin heterogeneous catalyst. The heterogeneous catalyst and food waste oil are added to flask, and preset amount of short-chain alcohol as ester exchange agent is added to obtain mixed solution. The mixed solution is heated at predetermined temperature for preset hours to obtain a mixture containing biodiesel and glycerol by-product. The obtained mixture is separated, distilled, dried and clarified to obtain biodiesel.