NOVELTY - Preparation of biodiesel comprises loading sulfuric acid loaded polyferric particle catalyst in reaction tower; preheating reaction tower at 70-100 degrees C; inputting methanol vapor; pumping liquid fatty acid at 110-160 degrees C; esterifying at 100-150 degrees C and 0-0.03 MPa; discharging generated biodiesel to bottom of reaction tower or reflowing biodiesel with acid value larger than 1 to upper part of catalyst layer until acid value is less than 1; and transporting biodiesel with acid value less than 1 to crude biodiesel storage tank.    USE - Preparation of biodiesel.    ADVANTAGE - The method solves problem that catalyst is separated from residual methanol. It provides continuous biodiesel production. The water generated in reaction procedure is vaporized at high temperature and is discharged out of reaction tower by residual methanol so that reaction proceeded. The conversation ratio of fatty acid is more than 98% and reaction time is short.    DETAILED DESCRIPTION - Preparation of biodiesel comprises loading sulfuric acid loaded polyferric particle catalyst in catalyst basket of reaction tower in 4-6 layers, in which each layer is 300-500 mm in height; preheating reaction tower at 70-100 degrees C; inputting methanol vapor; pumping liquid fatty acid at 110-160 degrees C, in which flow quantity ratio is based on mol ratio of methanol and liquid fatty acid which is (1.5:1)-(2.0:1); esterifying at 100-150 degrees C and 0-0.03 MPa; discharging generated biodiesel to bottom of reaction tower or reflowing biodiesel with acid value larger than 1 to upper part of catalyst layer until acid value is less than 1; transporting biodiesel with acid value less than 1 to crude biodiesel storage tank through pump; discharging water and residual methanol to methanol rectification tower through top part of tower; purifying hydrous methanol to 99% through methanol rectification tower and recycling product; and rectifying biodiesel with acid value less than 1 to obtain biodiesel finished product and biological pitch in decompression rectifying method. An INDEPENDENT CLAIM is included for special device for preparing biodiesel comprising methanol storage tank (1), esterification reaction tower (4), methanol vapor generator (3), fatty acid pre-heater (5), fatty acid storage tank (9), and crude biodiesel storage tank (10).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of special device for preparing biodiesel.    Methanol storage tank (1)    Methanol collecting tank (2)    Methanol vapor generator (3)    Esterification reaction tower (4)    Fatty acid pre-heater (5)    Methanol buffer tank (6)    Heat exchanger (7)    Methanol rectification tower (8)    Fatty acid storage tank (9)    Crude biodiesel storage tank (10)