NOVELTY - Preparing biodiesel oil using soap stock comprises mixing softening water with soap stock, stirring the solution, adding soap-stock into ethanol, refluxing ethanol, stirring obtained solution, adding sodium hydroxide solution into mixted solution and then complete saponifying, slowly adding sulfuric acid, sampling and detecting, stirring the solution, standing stirred solution, dividing the solution into 4 layers, adding softening water into refined rapeseed acidified oil, continuously stirring solution to form separate layer, and repeatedly washing layer by water and then dehydrating.    USE - The method is useful for preparing biodiesel oil (claimed).    ADVANTAGE - The method economically provides biodiesel oil with reduced energy consumption and environmental pollution.    DETAILED DESCRIPTION - Preparing biodiesel oil using soap stock comprises (1) mixing softening water with soap stock (20-30 mass%) at 80-90 degrees C, stirring the solution, adding soap-stock (50-100 mass%) into ethanol (96-99 mass%), refluxing the ethanol, stirring obtained solution until the solution is completely mixed, adding sodium hydroxide solution (30 mass%) into mixed solution and then complete saponifying and adjusting pH value to 10-11, insulating temperature at 70-75 degrees C for 30-50 minutes, keeping the mixed solution temperature at 70-75 degrees C until the ethanol is refluxed state, slowly adding sulfuric acid (96-98%) having a pH value to 2-3, sampling and detecting pH value to higher than 3, stirring the solution until pH remains unchanged, stopping stirring, standing the stirred solution for 30-40 minutes, dividing the solution into 4 layers, where a waste water is from bottom to top, first layer is sodium sulfate, the second layer is water, the third layer is rubber, and the fourth layer is acidified, (2) adding softening water into refined rapeseed acidified oil (18-22 mass%) at 90-95 degrees C, continuously stirring the solution to form separate layer, where the lower layer is impurity mixture, and repeatedly washing the layer by water and then dehydrating using a film evaporator to obtain standard raw material oil having a pH value to 7, and water content of 0.05 less than or equal to 0.2%, (3) mixing standard raw material oil, methanol and concentrated sulfuric acid to form mixed material, where the mass ratio of standard raw material oil, methanol and concentrated sulfuric acid is 100:30-35:1-2, heating the mixture until the methanol is refluxed, fully stirring and mixing obtained mixture under reacting condition for 30-35 minutes, sampling and detecting fatty acid methyl ester having a content of greater than or equal to 96%, separating the reaction mixture using separation device until the methanol is distilled off, and discharging the solid alkali catalyst to obtain rough biological diesel oil, and (4) washing rough biological diesel oil by hot water at 85-90 degrees C, removing residual catalyst and sulfuric acid, making the pH value to 7 and then dehydrating by thin film evaporator having water is less than or equal to 0.02%, introducing the rough biological diesel oil into discoloring tank, adding rough biological diesel oil 3-5% of clay, stirring and discoloring the mixture at 75-80 degrees C for 30-50 minutes, and filtering the decolored mixture to obtain 5-6 colored number of qualified biodiesel.