TECHNOLOGY FOCUS - BIOLOGY - Preferred Method: The method comprises: (i) adding corn bran and grit in a reaction kettle and adding Mortierella isabellina, as a microbial oil strain for the fermentation production, adding methanol, where the molar ratio of the microbial oil to methanol is 1:6, 1:12, 1:18, 1:24 or 1:12, low speed stirring for 5 or 15 minutes at 100 rotations/minute, adding sulfuric acid (5%, 2%, 3% or 4% of the microbial oil), continuing stirring at a speed of 400 rotations/minute at 65 degrees C, 55 degrees C, 70 degrees C or 60 degrees C for 1-4 hours, 3 hours, 2 hours or 1 hour, completing the stirring process, washing the oil phase to pH of 7, 6.7, 6.6 or 6.5 and heating for 30 minutes at 105 degrees C for removing water to obtain deacidified microbial oil; (ii) performing transesterification of microbial oil, which includes adding the deacidified microbial oil in the reaction kettle, adding methanol, where the molar ratio of the microbial oil to methanol is 1:6, 1:10, 1:12 or 1:14, dissolving potassium hydroxide (0.25%, 0.75%, 1% or 1.2% of the microbial oil) in the methanol, stirring at a speed of 400 rotations/minute and at 50 degrees C, 65 degrees C, or 55 degrees C for 1 hour or 2 hours, completing the stirring process, allowing the solution to stand for 2 hours, separating lower layer glycerin phase, adding hot water for washing (80% of the oil volume), and separating the hot water to obtain the water washed crude biodiesel; and (iii) refining the biodiesel by adding ethanol (3, 4, 5 or 6 times of the crude biodiesel), allowing the solution to stand for 5, 10 or 15 minutes after stirring, where the methyl ester enters the upper layer ethanol phase, separating the microbial oil which does not take part in the transesterification, which can be utilized again for the next preparation of the biodiesel, reducing the upper layer ethanol phase by distilling under vacuum at 0.1MPa and separating the ethanol at 40 degrees C to obtain the methyl ester-biodiesel and the ethanol can be circularly used.