TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The mass ratio of oil, lower aliphatic alcohol and catalyst is 1:O.2-0.5:0.01-0.05, respectively. The waste oil is hogwash oil, trench oil or frying oil. The lower aliphatic alcohol is methanol, ethanol, propanol or butanol. The separated catalyst is used for transesterification reaction in further preparation. Preferred Method: The preparation of biodiesel fuel comprises (i) dissolving sodium isethionate in ethyl acetate (20 times total wt. of sodium isethionate), adding maleic anhydride (10% of total wt. of ethyl acetate) to the above solution, where the ratio of maleic anhydride and sodium isethionate is 1:1, stirring for 2-14 hours, distilling to remove ethyl acetate, where the mixture comprises olefinic bond, sulfonic acid group and a carboxy group of monoester compound, dissolving monoester compound in ethanol (20 times total wt. of monoester compound), adding azobisisobutyronitrile initiator (0.5% of total wt. of monoester compound), heating and stirring at 70 degrees C, polymerizing for 4 hours to form oligomer, adding water, where the ratio of water and monoester compound is 1:1-10, adding tetraethyl orthosilicate in a dropwise manner, where the ratio of tetraethyl orthosilicate and the monoester compound is 1:1-8, heating and stirring at 70 degrees C for 12 hours to obtain a gel solid, naturally drying at room temperature, then drying in an oven at 80 degrees C for 12 hours to obtain solid acid composition, (ii) adding diatomite (1% of the total wt. of waste oil) to the waste oil, stirring at 70 degrees C for 2 hours, filtering, vacuum extracting and distilling the filtrate at 100 degrees C to remove moisture, (iii) performing transesterification reaction by electromagnetic stirring, a thermometer, reflux condenser pipe and a 100 ml three-necked flask for treating oil, lower aliphatic alcohols and catalyst of step (ii), where the mass ratio of oil, lower aliphatic alcohols and catalyst is 1:0.2-0.5:0.01-0.05, heat refluxing at 70 degrees C, and stirring for 12 hours, and (iv) filtering the crude product obtained in the step (iii), filtering the catalyst for further use, separating the filtrate, separating out the lower glycerol, adding the upper layer product to sodium bicarbonate solution with a mass fraction of 5-10%, where the amount of the upper product quality is 2-6 times of sodium bicarbonate solution, washing to neutral pH, removing unreacted free fatty acids, separating the upper organic phase with a separatory funnel, drying by using anhydrous magnesium sulfate, filtering to remove the drying agent, vacuum extracting and distilling.