NOVELTY - Preparing biodiesel, comprises e.g. performing pre-esterification of oil material by reacting methanol and oil material with concentrated sulfuric acid to make acid value below 3 mg potassium hydroxide/g, then performing ester exchanging by adding water to neutralize the pre-esterified material, dehydrating, drying and stirring it with 3% of sodium hydroxide methanol solution to make acid value in the system to reach 1 mg potassium hydroxide/g, and separating out glycerol methanol solution to obtain crude biodiesel, and performing multi-stage molecular distillation by using a film evaporator.    USE - The method is useful for preparing biodiesel.    ADVANTAGE - The method: has simple process and an esterification rate of greater than 98%; utilizes easily available materials and completes the whole process within 4 hours; and performs removing of color, smell and gas in a single step and thus shortens the process step and reduces the cost.    DETAILED DESCRIPTION - Preparing biodiesel, comprises (i) performing pre-esterification of oil material by adding methanol and the oil material in a molar ratio of 4:1-8:1 into a reactor, stirring by adding concentrated sulfuric acid (0.67 kg/minute), starting a vacuum pump to keep temperature in the reactor at 75 degrees C, reacting for 2 hours to make the acid value below 3 mg potassium hydroxide/g, and removing water and the unreacted methanol generated by the reaction, where oil material is of pre-treated animal fat and vegetable oil, then (ii) performing ester exchanging by adding water to neutralize and wash the pre-esterified material to adjust the pH of it to 5-7, then eliminating the water and a small amount of impurities through a separator, dehydrating and drying by a flash vessel until vacuum degree reaches to 0.098 Mpa, then pouring it into a reactor and raising the temperature to 75 degrees C, adding 3% of sodium hydroxide methanol solution under the vacuum degree of 0.098 Mpa, then reacting for 2 hours to make the acid value in the system to reach 1 mg potassium hydroxide/g, and allowing to stand for layer separation, separating out glycerol methanol solution to obtain crude biodiesel, and (iii) performing multi-stage molecular distillation by drying the crude biodiesel via flash vessel, then adding it into a film evaporator under 135 degrees C and vacuum degree of 200 Pa, then eliminating the lower components and steam, pumping the material into a second level molecular distillation process under 165 degrees C and vacuum degree of 10 Pa, then distilling out fatty acid methyl ester containing C16 and C18, and storing it in a tank, and portion which have not been distilled out is pumped into a third level molecular distillation process under 190 degrees C and vacuum degree of 1 Pa to distilling out fatty acid methyl ester containing C20 and C22, and storing it in a tank, and finally combining the distilled materials.