NOVELTY - A biodiesel is prepared from Jatropha fruit by using methanol/n-hexane mixed solvent as extraction and reaction reagent at the same time, using solid alkali potassium fluoride/ gamma -aluminum oxide as crystal; and finishing extraction of oil and catalytic ester exchange reaction in an integrated extracting and synthesizing reaction device.    USE - Method for preparing biodiesel (claimed).    ADVANTAGE - The method simplifies a pretreatment step and a subsequent pretreatment step, and realizes separation of the Jatropha fruit residue, catalyst, and product at the same time. It is capable of recycling the solid alkali catalyst, having a brilliant industrial application prospect.    DETAILED DESCRIPTION - Preparation of biodiesel from Jatropha fruit involves using methanol/n-hexane mixed solvent on volume ratio of 2-10:1 as extraction and reaction reagent, using solid alkali potassium fluoride/ gamma -aluminum oxide as crystal; using inert silicon oxide as filler; and finishing extraction of oil and catalytic ester exchange reaction in an integrated extracting and synthesizing reaction device to prepare the biodiesel. Jatropha fruit powder (20-40 meshes) is placed into an extracting reaction chamber (5). The solid alkali potassium fluoride/ gamma -aluminum oxide catalyst of 20-40 meshes and inert silicon oxide of 20-40 meshes are placed in a catalytic reaction chamber (2). The methanol/normal hexane mixed solvent is added into a solvent/product collecting chamber (1). The mass ratio of the Jatropha fruit powder to potassium fluoride/ gamma -aluminum oxide catalyst and methanol/n-hexane mixed solvent is 1:0.1-0.15: 40-60. The solvent/product collecting chamber is heated at 65-80 degrees C, and electromagnetically agitated and refluxed for 3.0-5.0 hours. The Jatropha fruit residue and the solid alkali catalyst are taken out after the reaction. After decompressing and distilling to remove the solvent from the solvent/product collecting chamber, the biodiesel is obtained. An INDEPENDENT CLAIM is included for an integrated extracting and synthesizing device.    DESCRIPTION OF DRAWING(S) - The drawing is a sectional view of an integrated extracting and synthesizing device.    Solvent/product collecting chamber (1)    Catalytic reaction chamber (2)    Siphon (3)    Extracting reaction chamber (5)