NOVELTY - Producing biodiesel comprises e.g. mixing N-methyl imidazole with chlorobutane, stirring, reacting, cooling, crystallizing to obtain a solid 1-butyl-3-methylimidazolium chloride ((BMM)Cl), mixing (BMM)Cl with sodium fluoroborate, adding acetone, stirring, reacting, filtering, collecting filtrate, decompressing, distilling, drying to obtain a ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ((BMM)BF4), mixing a micro-algae oil with alcohol, adding the (BMM)BF4, ultrasonically treating, transporting the reactant into a funnel, standing, decompressing, and distilling.    USE - The method is useful for producing biodiesel.    ADVANTAGE - The method is capable of producing biodiesel with high yield (90%) in a cost-effective and environment friendly manner.    DETAILED DESCRIPTION - Producing biodiesel by performing ultrasonic auxiliary ionic liquid catalytic reaction on microalgae grease, comprises uniformly mixing N-methyl imidazole with chlorobutane in a mole ratio of 1:2 under a nitrogen condition, stirring and reacting the mixture at 70-80 degrees C for 40-50 hours, cooling and crystallizing the reacted mixture to obtain a solid (BMM)Cl, mixing the obtained (BMM)Cl with sodium fluoroborate in a mole ratio of 1:1 to obtain mixed material, mixing the mixed material with acetone in a mass volume ratio of 1g:10-30 ml, stirring and reacting the mixture at room temperature for 18-24 hours to obtain a solid-liquid mixture, filtering the solid-liquid mixture, collecting the filtrate, decompressing, distilling and drying the collected fraction at a vacuum degree of 0.09-0.092 MPa and at 60-80 degrees C for 1-5 hours to obtain a ionic liquid (BMM)BF4, mixing a micro-algae oil with alcohol in a mole ratio of 6:10 to obtain oil-alcohol mixture, uniformly mixing the oil alcohol mixture with the obtained ionic liquid (BMM)BF4 in a mass ratio of 100:0.5-1.5, ultrasonically treating the mixture at a power of 100-300 W and a temperature of 40-60 degrees C for 20-50 minutes to obtain a reactant, transporting the obtained reactant into a separating funnel, standing the mixture at room temperature for 50-90 minutes to obtain a layered two phase solution, taking an upper layer solution from the layered two phase solution for decompressing, distilling and collecting all fractions to obtain a biodiesel, where the alcohol is methanol or ethanol.