NOVELTY - Preparing biodiesel and co-producing triacetin comprise taking dual-acid-type ionic liquid formed by alkyl sulfonic acid functionalized acidity quaternary ammonium salt cationic and Lewis acidity inorganic anions as a catalyst, where free fatty acid in acid-catalyzed high-acid-value oil is subjected to esterification reaction, and glyceride in acidity catalytic oil is subjected to ester exchange reaction to prepare biodiesel; and reacting an ionic liquid prepared from biodiesel and separating automatically after reaction and a glycerol mixture with acetic acid, and co-producing triacetin.    USE - Method for preparing biodiesel and co-producing triacetin (claimed).    ADVANTAGE - The selection range of raw material oil is much wider and economical. The catalyst ionic liquid has stable performances, has a good catalytic effect, and can be recycled.    DETAILED DESCRIPTION - Preparing biodiesel and co-producing triacetin comprise using Bronsted-Lewis acid ionic liquid by catalyzing the high-acid-value oil for preparing biodiesel by biocatalysis one-pot biodiesel oil and triacetin joint, where it uses alkyl sulfonic acid functionalized Bronsted acid quaternary ammonium salt cation and Lewis acidic inorganic anion comprising a Bronsted-Lewis acid type ionic liquid has a good agent, of a Bronsted in which acid bit good news for acid value oil is fatty acid esterification reaction of a Lewis acid bit of catalyzing oil ester exchange reaction for biodiesel preparing, separating and using the preparation of biodiesel fuel reaction of the ionic liquid, and a mixture with acetic acid, producing triacetin, where the high-acid-value grease (the acid value is 2). The mass ratio of 2-15 mg-potassium hydroxide/g and the methanol is 1:6-1:16, the catalyst ionic liquid oil is present at 2-10 mass%, the reaction is carried out at 50-140 degrees C for 6-12 hours, the prepared biodiesel oil, the prepared biodiesel oil self-separating the ionic liquid, and a mixture of glycerol and acetic acid mol ratio of 1:3.5-5 to toluene refluxing and reacting for 6-12 hours with water, adding glycerol mol lactose 6 times of acetic anhydride reflux reaction 1-4 hours, joint glyceryl triacetate.