TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The alkaline catalyst is potassium hydroxide or sodium hydroxide. The absorbing agent is white clay, which is added at 1-5, preferably 2 mass% of the mass of biodiesel. The step of absorbing, decolorizing, and removing soap is conducted for 15-30, preferably 20 minutes. The concentration of sulfuric acid is 98 mass%. The solid catalyst is particle-type sulfur oxide (SO42-)/MxOy solid ultra-strong acid catalyst, where particle-type base carrier is alumina (Al2O3), silicon dioxide (SiO2), or Al2O3-SiO2 composite oxide, and the active carrier is zirconium oxide (ZrO2), titanium dioxide (TiO2), tin oxide (SnO2), ZrO2-TiO2 composite oxide, or TiO2-SnO2 composite oxide.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The acid value of oil with high acid value is 20-160 mg potassium hydroxide/g. The crude glycerol has purity of 80-90 mass%. The mass ratio of crude glycerol to oil with high acid value is 0.15-0.35:1, preferably 0.25:1. The amount of solid catalyst is 0.1-0.5 or 0.02 mass% of the total mass of crude glycerol and oil with high acid value. The time for dripping crude glycerol is 20-45, preferably 30 minutes. Esterification is conducted at 170-230, preferably 210 degrees C and 2000-10000, preferably 5000 Pa for 2-5, preferably 3 hours. In step (A), the temperature of the cooling water in the reflux condenser is 30-80, preferably 50 degrees C; and the time for standing to separate into layers is 30-60, preferably 45 minutes. In step (A), centrifuging is conducted at 3000 revolutions/minute (rpm) for 10-15, preferably 15 minutes. The mass ratio of methanol to esterified oil is 0.2-0.25:1, preferably 0.2:1. The amount of alkaline catalyst is 0.5-1.2, preferably 1 mass% of the mass of esterified oil. Alcoholysis is conducted at 40-65, preferably 45 degrees C for 30-80, preferably 60 minutes. The step of heating to recycle methanol is carried out by heating at 110-120, preferably 120 degrees C under normal pressure. Neutralization is conducted by heating the glycerol layer to 80 degrees C and maintaining the reaction for 30 minutes. In step (D), the time for standing to separate into layers is 45 minutes, and centrifuging is conducted at 3000 rpm for 15 minutes.