NOVELTY - Preparing biodiesel by castor oil, comprises pre-processing the castor oil with hot water, filtering precipitate to remove impurities, removing moisture in the raw material by evaporation to obtain standard raw oil, adding solid alkali catalyst and methanol in the reactor and then adding the standard raw material oil to form mixed material, reacting materials in an ultrasonic cleaner, distilling to remove the methanol, filtering to remove the catalyst, layering and decoloring the rough bio-diesel by adding and stirring the alkaline decoloring agent, and filtering obtained product.    USE - The method is useful for preparing biodiesel by castor oil (claimed).    ADVANTAGE - The simple method provides biodiesel with high purity in a cost effective and less time consuming manner.    DETAILED DESCRIPTION - Preparing biodiesel by castor oil, comprises pre-processing the castor oil (20-35%) with hot water at 90-95 degrees C, and then continuously stirring, standing and depositing precipitate on a separating layer, filtering precipitate to remove impurities, removing moisture in the raw material by evaporation to obtain standard raw oil, performing ester exchange reaction by adding solid alkali catalyst and methanol in the reactor and then adding the standard raw material oil to form mixed material, where the mass ratio of the raw material oil, methanol and solid alkali catalyst is 100:70-80:1-3, mixing and reacting materials in an ultrasonic cleaner for 1-2 hours, detecting whether fatty acid methyl ester content is greater than 96%, distilling to remove the methanol, filtering to remove the catalyst, where rough bio-diesel is formed in an upper layer and byproduct glycerol is formed in a lower layer, layering and decoloring the rough bio-diesel and raising temperature to 60-70 degrees C, adding and stirring the alkaline decoloring agent (3-5 wt.%) to the rough biological diesel oil for 20-30 minutes, slowly raising the temperature to 100-105 degrees C, and filtering the obtained product.