NOVELTY - A biodiesel (19) is prepared by respectively feeding material oil and low-carbon alcohol to pre-heater; reacting the preheated materials in a membrane reactor (8) filled with catalyst by a mixer at 50-200 degrees C and 0.1-20 MPa, the pressure differential between exit of pipeline of reactor and exit of shell side being kept at 50-500 kPa; circulating the materials discharged by the pipeline to membrane reactor; and continuously feeding the materials from shell side to a decompression evaporation tower, and condensing the materials at top of flash evaporation tower to obtain low-carbon alcohol.    USE - Method for preparing biodiesel (claimed).    ADVANTAGE - The method allows for transesterification of grease and the separation process of products to be coupled in a membrane reactor filled with solid base and solid acid catalyst, thus simplifying the production process and reaction equipment, reducing the cost of equipment, reducing energy consumption, and improving efficiency. The yield of biodiesel is over 98%. The method can be continuously operated, which is environment-friendly and easy to amplify.    DETAILED DESCRIPTION - Preparing biodiesel (19) by static bed catalytic membrane reactor (8) comprises:    (A) respectively feeding material oil and low-carbon alcohol to pre-heater at a molar ratio of 1:3-1:16 at 50-70 degrees C;    (B) reacting the preheated materials in a membrane reactor filled with catalyst by a mixer at 50-200 degrees C and 0.1-20 MPa, the pressure differential between the exit of the pipeline of the reactor and the exit of the shell side being kept at 50-500 kPa;    (C) circulating the materials discharged by the pipeline to the membrane reactor; and    (D) continuously feeding the materials from the shell side to a decompression evaporation tower, condensing the materials at the top of the flash evaporation tower to obtain low-carbon alcohol, sending it to low-carbon storage jar, layering the materials at the bottom of the tower by cooling the material through a heat exchanger, and separating crude glycerin at the lower layer to a storage jar, in which the material at the upper layer is the biodiesel product.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic view showing production of biodiesel.    Methanol storage tank (1)    Methanol metering pump (2)    Methanol pre-heater (3)    Oil metering pump (4)    Oil pre-heater (5)    Emptying valve (6)    Oil storage tank (7)    Reactor (8)    Recycle pump (9)    Back pressure valve (10)    Heat exchanger (11)    Biodiesel (19)