NOVELTY - Preparing No.20 biodiesel from waste grease based on rectifying technique comprises selecting materials, hydrolyzing, distilling, esterifying, neutralizing, dealcoholizing, and rectifying to obtain the product; where the process specifically comprises: (a) selecting waste grease from animal and plants as the material; (b) separating the reactant to obtain rough fatty acid and glycerol aqueous solution; (c) feeding the rough fatty acid into the distillation column for distilling; (d) carrying out esterification; (e) adding pure base into the ester for neutralization; and (f) rectifying.    USE - The process is useful for preparing No.20 biodiesel from waste grease based on rectifying technique (claimed).    ADVANTAGE - The process overcomes the problems of low yield, few varieties, poor performance, and failure in adapting to low-temperature condition of the biodiesel prepared with traditional method, which provides a new way for producing new type of biodiesel with stable performance and high adaptability. Moreover, the product does not solidify at room temperature to -20 degrees C, and can be mixed with diesel freely without toxin or corrosion or damaging the machine member. Additionally, the product is biodegradable, environment-friendly, and can be used at any time with high boiling point and flashing point, which belongs to the non-hazardous material and is safe for storage and transportation with huge commercial prospect.    DETAILED DESCRIPTION - Preparing No.20 biodiesel from waste grease based on rectifying technique comprises selecting materials, hydrolyzing, distilling, esterifying, neutralizing, dealcoholizing, and rectifying to obtain the product; where the process specifically comprises: (a) selecting waste grease from animal and plants as the material, feeding it into clear water in a grease reverse hydrolytic column for hydrolysis; (b) keeping temperature, pressure and time of the hydrolytic reaction at 200-250 degrees C, 2.5-3.0 MPa, and 4-8 hours according to a ratio of oil to water of 1:0.4-0.5, separating the reactant to obtain rough fatty acid and glycerol aqueous solution; (c) feeding the rough fatty acid into the distillation column with residual vacuum pressure inside the column of lower than 2 mm of mercury column, distilling it at 200-250 degrees C to obtain mixture of oleic acid, linoleic acid, and by-products palmitic acid and stearic acid; (d) mixing the mixture of oleic acid and linoleic acid with methanol, adding sulfuric acid for esterification at 70-85 degrees C for 4-8 hours to obtain ester and water; (e) adding pure base into the ester for neutralization, removing the free acid, and removing methanol and recycling methanol; and (f) pumping the dealcoholized ester into a rectifying column with residual vacuum pressure inside the column of lower than 5 mm of mercury column, keeping temperature of the reaction at 200-230 degrees C, and removing the residual slag to obtain finished biodiesel.