NOVELTY - A biodiesel preparation method involves selecting raw materials, adding in a hydrolysis tower, followed by adding water to obtain crude fatty acid. The fatty acid is cooled, followed by adding stearic acid, and vacuum dehydrating, placing in a reaction kettle, adding sulfuric acid catalyst and methanol, carrying out esterification, adding methanol, and alkali solution, removing free acid and sodium sulfate, and filtering, removing methanol, and carrying out rectification. The residue is passed through a cooling tank, followed by discharging animal and vegetable oil to obtain biodiesel.    USE - Method for preparing biodiesel.    ADVANTAGE - The method enables preparing biodiesel with better product performance quality, high yield, low energy consumption and high industrial production value in a simple, environment-friendly, and economical manner.    DETAILED DESCRIPTION - A biodiesel preparation method involves selecting fluid waste animal and plant oil as raw materials, followed by adding in an hydrolysis tower, adding water to carry out hydrolysis reaction so as to obtain crude fatty acid and glycerine water solution, where the reaction temperature is 230-250 degrees C and pressure is 2-4MPa, the weight ratio of waste animal and plant oil, and water is 1:0.4-0.6, and the reaction time is not less than 7 hours. The fatty acid is cooled in a degreasing machine, followed by adding stearic acid to obtain fatty acid oil, and vacuum dehydrating at 120-140 degrees C. The dehydrated fatty acid oil placed in a reaction kettle, followed by adding 0.05 wt.% sulfuric acid catalyst and methanol not less than 15 wt.%, carrying out esterification at 110-130 degrees C for 3-4 hours. The fatty oil is added with 4-6 wt.% methanol, followed by adding 0.1-0.2 wt.% alkali solution, removing free acid and sodium sulfate and filtering. The methanol is removed in a pre-heating tank by pre-heating to 150-170 degrees C, followed by carry out rectification in pumping rectifying tower, where the vacuum residual pressure is less than 5 mmHg, and the rectifying tower temperature is 250-270 degrees C. The residue is passed through a cooling tank at 90-110 degrees C, followed by discharging animal and vegetable oil to obtain biodiesel.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow diagram illustrating a method for preparing biodiesel.'(Drawing includes non-English language text)'