NOVELTY - Preparation of biodiesel includes adding reactants comprising low carbon alcohol (e.g. methanol), oil (e.g. cottonseed oil or peanut oil) and cosolvent (e.g. tetrahydrofuran) to a flask, heating to 40-150 degrees C by water bath, adding supported solid alkali catalyst, and performing ester exchange reaction at 40-130 degrees C for 1-10 hours; stopping reaction, filtering supported solid alkali catalyst, vacuum steaming, and removing excess low carbon alcohol and cosolvent; and centrifugally separating the product to obtain an upper layer of biodiesel and a lower layer of byproduct glycerin.    USE - Method for preparing biodiesel (claimed).    ADVANTAGE - The method is simple, reduces environment pollution, decreases production cost and has ester exchange yield of up to 90%.    DETAILED DESCRIPTION - Preparation of biodiesel includes:    (A) adding reactants comprising low carbon alcohol, oil and cosolvent (5-80 wt.%, based on weight of oil) to a flask, heating to 40-150 degrees C by water bath, adding supported solid alkali catalyst (1-10 wt.%, based on weight of oil), and performing ester exchange reaction at 40-130 degrees C for 1-10 hours;    (B) stopping reaction, filtering supported solid alkali catalyst, vacuum steaming, and removing excess low carbon alcohol and cosolvent; and    (C) centrifugally separating the product to obtain an upper layer of biodiesel and a lower layer of byproduct glycerin.    The mole ratio of low carbon alcohol to oil is 3:1-20:1. The low carbon alcohol is methanol, ethanol, propanol, n-butyl alcohol, and/or amyl alcohol. The oil is soybean oil, transgene soybean oil, colza oil, transgene colza oil, palm oil, cottonseed oil, peanut oil, castor oil, sunflower seed oil, corn oil, olive oil, walnut oil, tung oil, sesame oil, rice bran oil, lard, beef fat, fish oil, chicken oil, and/or waste edible oil. The cosolvent is n-hexane, n-pentane, cyclohexane, n-heptyl, and/or tetrahydrofuran. An INDEPENDENT CLAIM is included for preparation of supported solid alkali catalyst by dissolving active component (including alkali metal, alkaline earth oxide, fluoride, carbonic acid compound, hydrate, and/or nitric acid compound) in solvent comprising water, ethanol, methanol, benzene, toluene, and/or ditoluene to prepare a solution comprising 3-40 wt.% active component and carrier; adding alkali metal or alkaline earth oxide (calcium oxide, magnesium oxide, molecular sieve, hydrotalcite, inactive alumina) to the solution for 0.5-24 hours, and drying in oven at 100-150 degrees C for 1-24 hours; and calcining and activating the active component in muffle at 300-700 degrees C for 1-24 hours.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic diagram of biodiesel preparation.