NOVELTY - Preparing biological diesel oil by ultrasonic cavitation and catalysis of swill-cooked dirty oil in the presence of weak acid metal salt, comprises: filtering the swill-cooked dirty oil, distilling and dehydrating to obtain raw material oil; adding the raw material oil and methanol into the reaction kettle with ultrasonic probe, adding metal salt of weak acid as a catalyst and stirring; heating and performing esterification reaction; finishing the reaction, cooling, filtering the catalyst to recycle and reuse; and vacuum distilling the filtered product to remove methanol, water and glycerin.    USE - The method is useful for preparing biodiesel oil by ultrasonic cavitation and catalysis of swill-cooked dirty oil in the presence of weak acid metal salt.    ADVANTAGE - The method improves the reaction efficiency, reduces the reaction time, improves the product rate, reduces environment pollution, simplifies subsequent processing process and reduces the production cost.    DETAILED DESCRIPTION - Preparing biological diesel oil by ultrasonic cavitation and catalysis of swill-cooked dirty oil in the presence of weak acid metal salt, comprises: (i) filtering the swill-cooked dirty oil, distilling and dehydrating to obtain raw material oil; (ii) taking the raw material oil and methanol in the molar ratio of 1-15:1, adding into the reaction kettle with ultrasonic probe, adding metal salt of weak acid as a catalyst and stirring at 1000-2000 revolutions/minute, where the amount of metal salt of weak acid accounts for 1-3% according to raw material oil and methanol mixture; (iii) heating to reaction temperature of 10 degrees C/minute speed, controlling the reaction temperature at 40-85 degrees C, starting ultrasonic wave generator for esterification reaction controlling the frequency at 10-60 KHz, and the power at 1-5 kw, and reacting for 10-120 minutes; (iv) finishing the reaction, cooling to 65 degrees C, filtering the catalyst to recycle and reuse; and (v) vacuum distilling the filtered product at vacuum degree of 0.08-0.09 MPa, temperature of 90-100 degrees C for 2 hours to remove methanol, water and glycerin, and processing.