NOVELTY - Preparation of biodiesel catalyzed by serial double-fixed bed solid catalyst comprises preheating oil and low carbon alcohol at mole ratio of 1:3-16; bringing the oil and low carbon alcohol in fixed bed filled with solid acid catalyst after pre-heating, mixing to obtain multi-phase reaction, esterifying the free fatty acid in the oil, and decreasing acid value of oil; bringing the resulting pre-processed material in the fixed bed filled with solid acid catalyst for ester exchange reaction to obtain biodiesel and glycerin mixture with fatty acid methyl ester as main component; and separating.    USE - Method of preparing biodiesel catalyzed by serial double-fixed bed solid catalyst (claimed).    ADVANTAGE - Ester exchange reaction is achieved with high efficiency. Acid value of oil is reduced fast. The method provides biodiesel with high purity. It has low investment cost. Glycerin in the product is easy to separate. Fine glycerin with purity of greater than 98.5% is obtained after decompression and rectification, thus improving use value of product.    DETAILED DESCRIPTION - Preparation of biodiesel catalyzed by serial double-fixed bed solid catalyst comprises:    (A) providing oil and low carbon alcohol in different containers and pouring them in a pre-heater through two metering pumps, where mole ratio of oil to the low carbon alcohol is 1:3-16;    (B) bringing the oil and low carbon alcohol in fixed bed filled with solid acid catalyst after pre-heating, mixing to obtain multi-phase reaction, esterifying the free fatty acid in the oil, and decreasing acid value of oil, where reaction temperature is controlled at 50-180 degrees C and reaction pressure is controlled at 0.1-2 MPa;    (C) bringing the resulting pre-processed material in the fixed bed filled with solid acid catalyst for ester exchange reaction to obtain biodiesel and glycerin mixture with fatty acid methyl ester as main component, where reaction temperature is controlled at 50-180 degrees C and reaction pressure is controlled at 0.1-2 MPa; and    (D) separating glycerin in the container through dividing layers, pouring biodiesel in upper layer and part of methanol in a fractionating tower by meter pump, and separating at normal pressure according to different boiling points of substances, to obtain low carbon alcohol at the top of tower, pure biodiesel at the middle part of tower, and oil having high boiling point and no impurity, at the base of tower.