NOVELTY - A biodiesel is prepared by mixing ester exchange reagent and animal-plant grease; and filtering and removing catalyst, distilling and removing excessive ester exchange reagent, and obtaining the biodiesel. The mixing of ester exchange reagent and animal-plant grease involves reacting for 2-18 hours at 30-450 degrees C in presence of catalyst, where mass ratio of ester exchange reagent to animal-plant grease is 1: 1-30:1, amount of catalyst is 1-30% of animal-plant grease, and pressure in reactor is 0.05-30 MPa. The ester exchange reagent is methyl carbonate and/or, diethyl carbonate.    USE - Method for preparing biodiesel (claimed).    ADVANTAGE - The method has no glycerin side product; avoids pollution of methanol steam at high temperature; and solves technical problem and economical problem in conventional preparation process of biodiesel. The catalyst is separated easily, does not need to be washed by water and it does not produce wastewater, thus can be used repeatedly and improving utilization rate of the raw material.    DETAILED DESCRIPTION - The biodiesel is prepared by mixing ester exchange reagent and animal-plant grease; and filtering and removing catalyst, distilling and removing excessive ester exchange reagent, and obtaining the biodiesel. The mixing of ester exchange reagent and animal-plant grease involves reacting for 2-18 hours at 30-450 degrees C in presence of catalyst, where mass ratio of ester exchange reagent to animal-plant grease is 1: 1-30:1, amount of catalyst is 1-30% of animal-plant grease, and pressure in reactor is 0.05-30 MPa. The ester exchange reagent is methyl carbonate, diethyl carbonate, dimethyl sulfate, diethyl sulfate, trimethyl phosphate, triethyl phosphate, dimethyl phosphate, diethyl phosphate, dimethyloxalate, diethyl oxalate, dimethyl malonate, diethyl malonate, dimethyl succinate, and/or diethyl succinate. The animal-plant grease is soybean oil, rap oil, palm oil, cotton seed oil, peanut oil, castor oil, sunflower seed oil, corn oil, olive oil, walnut oil, Tung oil, teal oil, rice oil, lard, butter, fish oil, chicken oil, and/or waste edible oil. The catalyst is solid acid catalyst, supported solid acid catalyst, solid alkali catalyst, supported solid alkali catalyst and/or organic alkali. The solid acid catalyst is silicon dioxide (SiO2), titanium oxide (TiO2), aluminum oxide (Al2O3), Al2O3-SiO2, TiO2-SiO2, and/or zirconium sulfate (ZrSO4). The supported solid acid catalyst is antimony pentafluoride-tantalum pentafluoride (SbF5-TaF5), SbF5-hydrogen fluoride-aluminum trifluoride (SbF5-HF-AlF3), platinum (Pt)-SbF5, zirconium oxide (ZrO2)/SiO2, sulfate ion (SO4)2-/TiO2, (SO4)2-/Al2O3, (SO4)2-/ZrO2, (SO4)2-/iron oxide (Fe2O3), (SO4)2-/cobalt ferrite (CoFe2O4), (SO4)2-/ZrO2-Fe3O4, (SO4)2-/zinc (Zn)-Fe2O4, (SO4)2-/ZrO2/cerium ion (Ce)4+, (SO4)2-/ZrO2-cerium oxide (CeO2), (SO4)2-/TiO2/lanthanum ion (La3+), and/or (SO4)2-/Ti-La-O. The solid alkali catalyst is magnesium oxide (MgO), calcium oxide, strontium oxide, barium oxide, zinc oxide, zirconium dioxide, cerium dioxide, lanthanum oxide, MgO-sodium hydroxide (NaOH), MgO-Na, Mg-aluminum (Al) composite oxide, alkali ion exchange resin, lithium methylate (LiOCH3), sodium methylate (NaOCH3), potassium methylate (KOCH3), rubidium methylate (RbOCH3), cesium methylate (CsOCH3), lithium ethylate (LiOCH2CH3), NaOCH2CH3, KOCH2CH3, RbOCH2CH3, CsOCH2CH3, lithium isopropanol, sodium isopropanol, potassium isopropanol, rubidium isopropanol, cesium isopropanol, lithium n-butanol, sodium n-butanol, potassium n-butanol, rubidium n-butanol, cesium n-butanol, sodium tert-butanol, potassium tert-butanol, rubidium tert-butanol, cesium tert-butanol, Mg(OCH3)2, Ca(OCH3)2, Sr(OCH3)2, Ba(OCH3)2, Mg(OCH2CH3)2, Ca(OCH2CH3)2, Sr(OCH2CH3)2, Ba(OCH2CH3)2, magnesium glycol, calcium glycol, strontium glycol, barium glycol, magnesium isopropanol, calcium isopropanol, strontium isopropanol, barium isopropanol, magnesium n-butanol, calcium n-butanol, strontium n-butanol, barium n-butanol, magnesium tert-butanol, calcium tert-butanol, strontium tertiary butanol, and/or barium tert-butanol. The active component of supported solid acid catalyst is oxide, fluoride, carbonic-acid compound, hydroxide, and/or nitric acid compound. The carrier is CaO, MgO, molecular sieve, brucite, ion exchange resin, and/or active aluminum oxide. The organic alkali is guanidine, and/or cyclo-guanidine.