NOVELTY - Preparing biodiesel of auxiliary compatibilizer comprises (i) taking grease in reaction kettle, adding short-chain alcohol and fatty acid methyl ester, and subjecting to high-speed stirring to obtain uniform emulsion liquid, (ii) adding weak alkaline substance to emulsion liquid, and stirring under high speed, (iii) preparing methanol solution of potassium hydroxide or sodium hydroxide, adding into emulsion liquid, and carrying out reflux reaction, (iv) cooling, separating, and removing lower layer to obtain biodiesel crude product, (v) distilling and (vi) subjecting to molecular distillation.    USE - The method is useful for preparing biodiesel of auxiliary compatibilizer (claimed).    ADVANTAGE - The method: has simple preparation process, mild conditions, simple post-treatment process and low production cost; and is suitable for large-scale application, and can easily achieve industrialization.    DETAILED DESCRIPTION - Preparing biodiesel of auxiliary compatibilizer comprises (i) taking grease in a reaction kettle with a high-speed stirring device, then adding short-chain alcohol and fatty acid methyl ester, and subjecting to high-speed stirring at room temperature for 1-4 hours to obtain uniform emulsion liquid, (ii) adding weak alkaline substance to the emulsion liquid, and stirring under high speed for 1-4 hours, (iii) dissolving potassium hydroxide or sodium hydroxide in methanol to obtain 8-10% methanol solution of potassium hydroxide or sodium hydroxide, which is taken as a catalyst, adding into the emulsion liquid, and carrying out reflux reaction for at 50-120 degrees C for 0.5-4 hours, (iv) after the reaction is finished, cooling the reaction material to 40 degrees C, allowing to stand for separating, removing lower layer of glycerol and other by-product to obtain biodiesel crude product, (v) distilling the biodiesel crude product under atmospheric pressure, and recovering excess short-chain alcohol, and (vi) subjecting to molecular distillation to obtain the biodiesel, where the biodiesel comprises 100 wt.% grease, 20-120 wt.% short-chain alcohols, 1-10 wt.% fatty acid methyl ester, 0.5-4 wt.% catalyst, and 0.1-5 wt.% weak alkaline buffer substance.