TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In the method above, the crude product of biodiesel in (a) is esterifiable reaction product after performing the esterifiable reaction to the raw material oil of seed fat, animal oil, microbial grease or swill-cooked dirty oil with the methanol; the film evaporator comprises falling film evaporator and rotating scrapper type film evaporator. Before using the film evaporator to degas and distill the crude product of biodiesel, it further comprises the step of holding the crude product of biodiesel or eccentrically removing the byproduct with high speed, obtaining the crude methyl ester. Step (a) further comprises: (i) degassing the crude methyl ester via the falling film evaporator; and (ii) distilling the degassed product via the rotating scrapper type film evaporator, and obtaining the biodiesel with the methyl ester content of more than 96%. Step (b) of performing alkaline soil deacidifying to the product after the isolating process in (a) comprises: (i) detecting the acid value of biodiesel with the methyl ester content of more than 96%; (ii) calculating the usage of saturated alkaline solution according to the acid value when the alkaline matter is under the temperature of 20-90 degrees C, calculating the usage of the alkaline soil in the ratio of 1:0.5-5 of the alkaline soil according to the acid value; (iii) uniformly mixing the alkaline soil and the saturated alkaline solution on the condition of warm preservation and forming the alkaline soil deacidifying agent; (iv) uniformly mixing the alkaline soil deacidifying agent in (iii) and the biodiesel with the methyl ester content of more than 96% on the condition of warm preservation, stirring and reacting for 20-40 minutes; and (v) removing the solid matter, obtaining the purified biodiesel with the acid value in accordance with the national standard. The saturated alkaline solution in (b) is sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), sodium sulfite (Na2SO3), sodium hydroxide (NaOH) or potassium hydroxide (KOH) saturated solution. The temperature of (b) is 30-90 degrees C, the rotation speed of stirring in (d) is 100-1000 rotations per minute (rpm); it needs to keep warm and hold for 5-60 minutes after stopping reaction. Step (e) of deacidifying the removed solid matter comprises the method of acid-processing it for 15-60 minutes at 30-60 degrees C with rotation speed of 100-500 rpm, where the acid is sulfuric acid or hydrochloric acid.