NOVELTY - Preparing biodiesel using microbial oil as a raw material comprises: e.g. (i) adding corn bran, grit, Mortierella isabellina and methanol in a reaction kettle, stirring, adding sulfuric acid, continuing stirring and heating to obtain deacidified microbial oil; (ii) adding the deacidified microbial oil and methanol in the reaction kettle, dissolving potassium hydroxide, stirring, separating lower layer, adding hot water for washing and separating to obtain the crude biodiesel; and (iii) adding ethanol and reducing the upper layer by distilling to obtain the methyl ester-biodiesel.    USE - The method is useful for the preparation of biodiesel using microbial oil as a raw material.    ADVANTAGE - The method: produces biodiesel according to the national standard with a yield of 71.56-93.20% and high industrial value; is environmental friendly, simple, easy to control, economical, and suitable for industrial production; has mild reaction conditions; and deacidifies the microbial oil, such that the acid value of the microbial oil is 1.58-1.90 mg potassium hydroxide/g.    DETAILED DESCRIPTION - Preparing biodiesel using microbial oil as a raw material 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 1:6-24, low speed stirring for 5-15 minutes at 100 rotations/minute, adding sulfuric acid (2-5% of the microbial oil), continuing stirring at a speed of 400 rotations/minute at 55-70 degrees C for 1-4 hours, completing the stirring process, washing the oil phase to pH of 6.5-7 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 1:6-14, dissolving potassium hydroxide (0.25-1.2% of the microbial oil) in the methanol, stirring at a speed of 400 rotations/minute and at 50-70 degrees C for 1-3 hours, completing the stirring process, allowing the solution to stand, separating lower layer glycerin phase, adding hot water for washing, and separating the hot water to obtain the water washed crude biodiesel; and (iii) refining the biodiesel by adding ethanol (3-6 times of the crude biodiesel), allowing the solution to stand for 5-15 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.