NOVELTY - Preparing biodiesel, comprises e.g. pre-heating animal fat, plant fat, and/or swill oil in a kettle, adding methanol, concentrated sulfuric acid, then opening a vacuum pump, maintaining the kettle to in negative pressure state and temperature to 75-80 degrees C and reacting, then heating the material after pre-esterification, rinsing with water, dehydrating and drying with a flash evaporator, pumping the obtained material in the kettle, and heating to 75 degrees C, adding sodium hydroxide-methanol solution, and reacting to obtain crude biodiesel, and then removing low-component substance and water vapor.    USE - The method is useful for preparing biodiesel.    ADVANTAGE - The method: is simple; shortens the whole esterification time by 1/2; and has an ester exchange rate of 93%; completes the processes of dehydrating, color removing and gas removing at the same time, and thus it shortens the process; reduces energy consumption and cost; and produces the product with stable quality and the technical indexes, which reaches or exceeds the 0# diesel national standard.    DETAILED DESCRIPTION - Preparation of biodiesel, comprises: (i) taking pre-treated animal fat, plant fat, and/or swill oil as raw material oil, pre-heating the raw material oil in a kettle to 60 degrees C, adding methanol to the kettle in a molar ratio of methanol to oil of 4:1-8:1, while stirring, adding concentrated sulfuric acid that is 0.03% of the raw material oil in amount and controlling the adding speed to 0.67 kg/minute, then opening a vacuum pump, maintaining the kettle to in negative pressure state and temperature to 75-80 degrees C to react for 2 hours, enabling acid value in the reaction system to less than 3 mg potassium hydroxide/g, and then extracting water generated in the reaction and methanol that does not take part in reaction in vacuum state; (ii) (a) heating the material after pre-esterification to 60-65 degrees C, and rinsing with water until its pH is 5-7, (b) removing water and a small amount of impurity via a separator, then (c) dehydrating and drying the obtained material with a flash evaporator with vacuum degree of 0.098 Mpa, (d) pumping the material in the kettle, and heating to 75 degrees C in a condition of stirring, uniformly adding sodium hydroxide-methanol solution with a concentration of 3%, and maintaining negative pressure to 0.098 Mpa to react for 2 hours, and reducing acid value of the system to 1 mg potassium hydroxide/g, and (e) standing and layering the mixture, and separating the glycerol-methanol solution at lower layer to obtain crude biodiesel; and (iii) (f) drying the crude biodiesel with flash evaporator, then placing it in a thin film evaporator, adjusting temperature to 135 degrees C and vacuum degree to 200 Pa, and removing low-component substance and water vapor, (g) pumping the crude biodiesel in a two-grade molecular distiller, and adjusting the temperature to 165 degrees C and vacuum degree to 10 Pa to distil fatty acid methyl ester containing 16C or 18C, and storing the fatty acid methyl ester in a tank, then (h) pumping the material that is not evaporated in the step (g) into a three-grade molecular evaporator, and adjusting temperature to 190 degrees C and vacuum degree to 1 Pa to distil fatty acid methyl ester containing 20C, and storing the fatty acid methyl ester in the tank, and (I) merging the distilled substances in the steps (g) and (h).