NOVELTY - Producing biodiesel from mixed fatty acid comprises mixing waste of animal and plant oil, soap stock, acidified oil, and edible oil recovery, heating and stirring, hydrolyzing for over 6 hours at 220 degrees C and pressure of 20 MPa, sampling and cooling and placing in settling tank, fractionating essential oil acid and mixed fatty acid; esterifying for 1.5 hours at 100-170 degrees C and pressure of 4-10 MPa and hourly space velocity of 0.5 hours, recovering methanol, adding ester phase substance in methanol recovery tower, and entering flash fractionating tower for fractionating biodiesel finished product.    USE - The process is useful for producing biodiesel from mixed fatty acid (claimed).    ADVANTAGE - The economic effect of biodiesel enterprise is improved, and the combined cost of biodiesel production is reduced; where the esterification time of vibration tube type continuous esterifying process is short that is only 1.5 hours. Moreover, a mass of energy source is saved; the yield of fatty acid methyl ester is as high as over 98%; and the quality of product reaches the request of national BD100 standard.    DETAILED DESCRIPTION - Producing biodiesel from mixed fatty acid comprises mixing waste of animal and plant oil, soap stock, acidified oil, and edible oil recovery, and removing mechanical impurity, adjusting the pH value to 3-4, heating directly with steam and stirring, feeding to hydrolyzing tower with water content of 40-50%, hydrolyzing for over 6 hours at 220 degrees C and pressure of 20 MPa, sampling and cooling when the acid value is tested to be over 180 mg potassium hydroxide (KOH)/g and placing in settling tank, discharging lower glycerol and water, dehydrating the upper material, fractionating essential oil acid and mixed fatty acid in vacuum distillation tower after dehydrating; adding esterifying agent in mixed fatty acid, esterifying for 1.5 hours at 100-170 degrees C and pressure of 4-10 MPa and group liquid hourly space velocity of 0.5 hours through three groups of vibration tube type continuous esterifying devices, recovering coarse methanol, pumping in methanol wash tower and washing with methanol, adjusting to esterifying agent with methanol phase containing acid, adding ester phase substance in methanol recovery tower for distilling off residual methanol, and entering flash fractionating tower for fractionating biodiesel finished product.