NOVELTY - Biodiesel is continuously prepared by pretreating a raw material by filtrating, and continuously vacuum dehydrating or flash evaporation dehydrating; passing the raw material oil through three reactors, where methanol is reacted with the raw material using a catalyst; using a superfluous methanol as water taking agent to make the water produced by esterification reaction be ushered out the reaction system at the same time; refining by entering a fatty acid methyl ester vacuum rectification tower (4); vacuum rectifying a fatty acid methyl ester; and rectification recovering methanol.    USE - Method for continuously preparing biodiesel.    ADVANTAGE - The production is separated using reduced pressure rectification process, the product is of high purity and good yield, and the reaction process has little wastes.    DETAILED DESCRIPTION - Continuous preparation of biodiesel comprises pretreating a raw material comprising removing mechanical impurities by filtrating, and continuously vacuum dehydrating or flash evaporation dehydrating, to make the containing water quality fraction be declined to below 0.5%, where the peracid value animals and plants oil is adopted by the raw material oil, and the acid value of raw material oil is 10-200 mg KOH/g; passing the raw material oil through a first reactor (1a), a second reactor (1b), and a third reactor (1c), where the methanol continuously adverse current passes through, and is reacted with the raw material when catalyzed by a catalyst, the temperature of the reactor is 105-230 degrees C, pressure of 0.8-2.5 MPa, the dosing of the catalyst is 1-20 wt.% of the raw material oil, the mass ratio of the methanol and the dosing of the catalyst is 0.3:1-2:1, the average residence time of the raw material oil in reactor is 1-6 hours; the superfluous methanol is used as water taking agent to make the water which is produced by esterification reaction be ushered out the reaction system at the same time, enters the next methanol rectification tower to be recycled for being used; the reaction production enters a fatty acid methyl ester vacuum rectification tower (4) to be refined through a gas and liquid segregator (2a), thus obtaining biodiesel and glycerol by-product; vacuum rectifying a fatty acid methyl ester, the liquid phase production which is from the gas and liquid segregator entries the fatty acid methyl ester vacuum rectification tower, siding is produced to get biodiesel product oil, the rectification pressure is 0.1-5 kPa; and rectification recovering methanol, superfluous methanol carries secretly the water which is produced by esterification reaction entering a methanol rectification tower (3).    DESCRIPTION OF DRAWING(S) - The drawing is a flowchart of the method for continuously preparing biodiesel.    First reactor (1a)    Second reactor (1b)    Third reactor (1c)    Gas and liquid segregator (2a)    Methanol rectification tower (3)    Fatty acid methyl ester vacuum rectification tower (4)