NOVELTY - Preparing biodiesel joint neutral oil from waste oil involves: pre-processing raw material using swill oil; using total fatty acids for esterification reaction to obtain second level condensing methanol, then decompressing and distilling to obtain the biological diesel oil.    USE - For preparing biodiesel joint neutral oil from waste oil (claimed).    ADVANTAGE - The method uses uniform ratio of raw material which ensures the subsequent technology parameter of consistency, reduces the rectifying dimethyl ether from methanol by weight (namely reducing loss), reduces energy consumption, reduces waste water discharge.    DETAILED DESCRIPTION - Preparing biodiesel joint neutral oil from waste oil involves: pre-processing raw material using swill oil by firstly carrying out a melting process, melting the combined into a water washing pot by a pump for washing and removing the impurity, heating to 60 degrees C, adding mass fraction 1% to the phosphoric acid, stirring and raising the temperature to 75 degrees C for hydration degumming, to generate 3% waste water, water, and oil separation through standing and layering, separating the oil and water, carrying out drying and dehydrating system and in a vacuum state, where the hydrated water is washed with water and the waste water processing station to process the oil tank pump oil filter, in a dry clay by adsorption to remove a pigment, removing oil continuously and deacidifying and deodorizing tower via the acid value of less than 2 of the neutral oil in 260 degrees C and 200 Pa condition can be obtained, if need to obtain the neutral oil as a value needs to be the water vapor at 20kg/hours degrees centigrade in the condition so as to improve the partial pressure of residual fatty acid so as to make the removing; taking the total fatty acid for esterification reaction, to a neutral oil to carry out the ester exchange reaction, where the esterification used in the reaction is greater than or equal to 70 mass.% concentrated sulfuric acid to adding, 0.2 wt.% ironbased catalyst of fatty acid into a 0.2 wt.%, where mol ratio of the methanol and fatty acid is 4:1, namely 1 ton of fatty acid to the 457 kg methanol and the reaction consumption is 114 kg, the rest of 343 kg of methanol and 64 kg of water generated by the reaction into the methanol rectification tower, a crude methanol to obtain 99.5% of methanol containing 0.5 mass.% methanol waste water rectifying tower refining, where the water enters the waste water processing station to process after collecting by the ester exchange reaction of methanol exchanging recovery methanol with fresh methanol mixture, ester is an ester exchange recycling methanol and fresh methanol is 1:1, mol ratio of methanol and grease is 6:1, where the reaction there is no waste water, generated by the reaction of glycerol can be quickly separated by sedimentation method, rough and a removing process and sale as by-product, two of the reaction produce is 0.2 mass.% methanol flash removing methanol, to obtain second level condensing the methanol, and then decompressing and distilling to obtain bio-diesel.