NOVELTY - Biodiesel is prepared by esterifying high acid oil, alkaline catalyst A, and crude glycerin, stirring, refluxing in vacuum, controlling temperature at 30-80 degrees C using cooling water, vacuum pumping generated water, condensing and refluxing glycerin, performing alcoholysis, adding methanol to alkaline catalyst B, carrying out dealcoholization process in vacuum distillation to obtain biodiesel layer, collecting fraction to obtain refined biodiesel oil, adding glycerin layer to concentrated sulfuric acid, and heating.    USE - Preparation of biodiesel (claimed).    ADVANTAGE - Preparation process is low in cost, has high catalytic activity and catalyzing efficiency, and no waste discharge, saves energy, and improves yield of biodiesel.    DETAILED DESCRIPTION - Biodiesel is prepared by esterifying high acid oil, alkaline catalyst A, and crude glycerin, stirring, refluxing in vacuum to obtain homogeneous mixture, controlling temperature at 30-80 degrees C using cooling water, vacuum pumping generated water, condensing and refluxing glycerin, performing alcoholysis on homogeneous mixture, adding methanol to alkaline catalyst B in which alcoholysis reaction layer comprises upper layer of biodiesel and lower layer of glycerin, carrying out dealcoholization process in vacuum distillation to obtain biodiesel layer, collecting fraction to obtain refined biodiesel oil, adding glycerin layer to concentrated sulfuric acid, and heating to obtain crude glycerin in which crude glycerin is continuously used as raw material in esterification reaction to achieve raw material utilization and remaining crude glycerin is obtained through esterifying acid oil, glycerin, and sulfuric acid, mass ratio crude glycerin:high acid oil is 0.20-0.60:1, total mass of high acid oil is 0.1-0.8 mass% of alkaline catalyst A, mass ratio methanol:homogeneous mixture is 0.16-0.40:1, and mass of alkaline catalyst B is 0.5-1.2 mass% homogeneous mixture.    DESCRIPTION OF DRAWING(S) - The drawing shows a process flow diagram for preparation of biodiesel. (Drawing includes non-English language text).