NOVELTY - Biodiesel is produced by heating animal or plant oil in a reactor at 32-37 degrees C, adding 0.2-0.4 wt.% citric acid, stirring, adding 0.2-0.4 wt.% saline solution, heating, depositing for 3.5-4.5 hours, and removing water, colloidal material and crude glycerin to obtain pre-esterified animal or plant oil; adding 0.5-2 wt.% concentrated sulfuric acid into 18-22 wt.% methanol and stirring, adding the mixture into the pre-esterified animal or plant oil, heating, and carrying out back flow reaction for 2.8-3.2 hours; and fractionating esterified material to obtain methanol and fatty acid methyl ester.    USE - Method for producing biodiesel (claimed).    ADVANTAGE - The method does not need to select material with good liquidity, and obtains biodiesel having low viscosity, light color, low solidifying point, and high cetane number. The carbon chain is reduced to 18-20C, the content of free acid is less than 0.5%, and the yield of fatty acid methyl ester is more than 90%.    DETAILED DESCRIPTION - Production of biodiesel comprises:    (A) putting animal or plant oil into a reactor, and heating at 32-37 degrees C; adding 0.2-0.4 wt.% citric acid into animal or plant oil; stirring for 15-20 minutes at 48-52 degrees C; adding 0.2-0.4 wt.% saline solution; stirring for 18-22 minutes, heating at 88-92 degrees C, and then stopping the heating step; depositing for 3.5-4.5 hours; and removing water, colloidal material and crude glycerin deposited at the bottom of the reactor to obtain pre-esterified animal or plant oil;    (B) adding 0.5-2 wt.% concentrated sulfuric acid into 18-22 wt.% methanol and stirring for 18-22 minutes; adding the mixture into the pre-esterified animal or plant oil in the reactor; stirring for 18-22 minutes and heating at 72-90 degrees C; opening valve between reactor and back flow device, carrying out back flow reaction for 2.8-3.2 hours, and then stopping the heating step; when temperature of material in the reactor reduces to 35-40 degrees C, adding mixture formed by weak alkali and methanol; and heating at 72-130 degrees C, and back flow reacting for 3.8-4.2 hours to obtain esterified material, where during twice back flow reaction process, pressure in the reactor is 0.1-0.5 MPa;    (C) putting esterified material into fractionating tower with lower part having heating device, middle part having adsorption filling device, and upper part having condensation device; and fractionating to obtain methanol at less than 70 degrees C, and fatty acid methyl ester at 70-360 degrees C;and    (D) decoloring, separating, and filtering the fatty acid methyl ester.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic view of an apparatus for producing biodiesel.