NOVELTY - Preparing biodiesel comprises: (a) carrying out ester exchange reaction and refining product; and (b) separating crude glycerol: exhausting glycerol generated by ester exchange reaction and the unreacted raw material from the bottom of the ester exchange reactor before entering a glycerol separator, inletting the glycerol separated from the lower layer into a glycerol recycling system to be recycled; and returning the unreacted glycerol separated from the upper layer to the ester exchange reactor to be recycled.    USE - The process is useful for preparing biodiesel (claimed).    ADVANTAGE - In the process, the biodiesel can be timely separated out of the reactor, which promotes the reaction to be carried out in the positive direction; and the methanol gas is circulated, which reduces the energy consumption. Moreover, the methanol gas is taken as reactant in the reaction system and plays a stirring role; a mesoporous solid catalyst is used; the product is easily separated; it is simple to recycle and purify the glycerol; and the product is environment-friendly. Additionally, the process is simple in technique and wide in the range of raw material application; and it can efficiently and continuously produce the biodiesel.    DETAILED DESCRIPTION - Preparing biodiesel comprises: (a) carrying out ester exchange reaction and refining product: continuously adding oil fat in an ester exchange reactor, inletting methanol steam generated by a methanol gasification device after hot bath and heating from a gas distributing plate mounted at the bottom of an ester exchange reactor to a reactor with mesoporous solid base catalyst, carrying out ester exchange reaction between oil fat and methanol steam under the role of the mesoporous solid base catalyst in a fixed bed; taking reaction product biodiesel along with the remaining methanol steam out of the reactor and entering a condenser; exhausting the biodiesel in the form of liquid from the bottom of the condenser; where the surface pressure of ester exchange reactor is 0-0.015 MPa, and the temperature is 220-250 degrees C; the surface pressure of methanol gasification device is 0-0.02 MPa, and the temperature is 60-70 degrees C; the hot bath temperature is 200-250 degrees C; the temperature of condenser is 100-120 degrees C; and the flow of methanol steam is 80-100 m3; and (b) separating crude glycerol: exhausting glycerol generated by ester exchange reaction and the unreacted raw material from the bottom of the ester exchange reactor before entering a glycerol separator, inletting the glycerol separated from the lower layer into a glycerol recycling system to be recycled; and returning the unreacted glycerol separated from the upper layer to the ester exchange reactor to be recycled.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the system used in preparing biodiesel (Drawing includes non-English language text).