NOVELTY - Preparation of a biodiesel comprises placing oil, short-chain alcohol, and an acidating solid catalyst in a reaction kettle, sealing the kettle, removing oxygen in the reaction system by inletting nitrogen, and inletting a gas cosolvent; heating the kettle at 110-180 degrees C, and carrying out an ester exchange reaction for 5 minutes to 2 hours at 0.1-7 MPa; and filtering the reaction mixture, recycling the filtered catalyst, decompressing and distilling the filter liquor, obtaining alcohol, stewing the distilled remaining liquor, and delaminating.    USE - Method for the preparation of a biodiesel (claimed).    ADVANTAGE - The cosolvent improves the catalytic activity of the catalyst, reduces reaction temperature, and enhances reaction rate. Post treatment of the biodiesel is simple and free from generating three-waste pollution. The catalyst can be easily recycled without corroding the apparatus used.    DETAILED DESCRIPTION - Preparation of a biodiesel comprises placing oil, short-chain alcohol, and 0.1-5% acidating solid catalyst in a reaction kettle, sealing the kettle, removing oxygen in the reaction system by inletting nitrogen, and inletting a gas cosolvent which enables the pressure in the kettle to be 0.5-4 MPa; heating the materials in the kettle at 110-180 degrees C, and carrying out an ester exchange reaction for 5 minutes to 2 hours at 0.1-7 MPa; and filtering the reaction mixture, recycling the filtered catalyst, decompressing and distilling the filter liquor, obtaining alcohol, stewing the distilled remaining liquor, and delaminating, where the upper oil phase is the biodiesel, and the lower is glycerin. The mole ratio of short-chain alcohol to oil is 3-27:1. The oil comprises soybean oil, transgenic soybean oil, colza oil, transgenic colza oil, coconut oil, cotton seed oil, castor oil, tung oil, palm oil, peanut oil, persic oil, peach kernel oil, sunflower oil, linseed oil, corn oil, olive oil, lard, butter, fish oil, catering industry recovered oil, industrial waste oil, swill oil, trench oil, or acid oil. The acidating solid catalyst is sulfate (SO 42-)/M xO ytype, SO 42-/titanium dioxide (TiO 2)-zirconium dioxide (ZrO 2), SO 42-/ZrO 2-ferric oxide (Fe 2O 3), SO 42-/TiO 2-Fe 2O 3composite, in which the SO 42/M xO ytype acidating solid catalyst comprise zirconium sulfate tetrahydrate (Zr(SO 4) 2.4H 2O), zirconium sulfate (Zr(SO 4) 2)/AC, Zr(SO 4) 2/aluminum oxide (Al 2O 3), Zr(SO 4) 2/silicon dioxide (SiO 2), copper sulfate, iron sulfate (Fe 2(SO 4) 3), zinc sulfate (ZnSO 4), cobalt (III) sulfate (Co 2(SO 4) 3), manganese sulfate (MnSO 4), copper chloride, ferric chloride, zinc chloride, cobalt chloride, manganese chloride, SO 42/zirconium oxide (ZrO 2), SO 42-/TiO 2, SO 42-/zinc oxide, SO 42-/Fe 2O 3, SO 42-/manganese oxide, SO 42-/Al 2O 3, or SO 42-/silicon dioxide. The cosolvent is carbon monoxide, carbon dioxide, methane, ethane, propane, butane, or pentane.    M, x, y=not defined.