NOVELTY - A hydrophobic solid base catalyst (I) is prepared by (1) performing activated carbon pretreatment to obtain modified activated carbon carrier; and (2) treating the modified activated carbon carrier with sodium hydroxide-ethanol mixed liquid (A), treating with magnesium chloride-ethanol mixed liquid (B), drying, calcining and then cooling to obtain the catalyst (I).    USE - Hydrophobic solid base catalyst (I) used for synthesizing biodesel from grease containing water and free fatty acid (claimed).    ADVANTAGE - The catalyst (I) has high catalytic activity and is configured with hydrophobic property which can effectively prevent the effect of water and free fatty acid in the reaction system during biodiesel synthesis. It contains MgO as active component which is insoluble in water, thus it can be utilized directly for synthesizing biodiesel from grease containing water and fatty acid.    DETAILED DESCRIPTION - A hydrophobic solid base catalyst (I) is prepared by (1) performing activated carbon pretreatment to obtain modified activated carbon carrier; (2) treating the modified activated carbon carrier with sodium hydroxide-ethanol mixed liquid (A); and (3) treating with magnesium chloride-ethanol mixed liquid (B), drying, calcining and then cooling to obtain the catalyst (I). The activated carbon pretreatment in step (1) is performed by (a) adding activated carbon into distilled water at 50 g/L, mixing at 400 revolutions/minute (rpm), heating at 100 degrees C for 4-5 hours, filtering the mixture to obtain the activated carbon; (b) repeating step (a) for 2 times, and then drying the activated carbon to a constant weight; (c) adding the activated carbon into hexamethyldisiloxane at 10 g/L, mixing at 300 rpm and soaking 12 hours, controlling the temperature to 50 degrees C, filtering, and vacuum drying the solid at 70 degrees C for 3 hours to obtain the modified activated carbon carrier. Step (2) is performed by adding sodium hydroxide into absolute ethyl alcohol at 100 g/L, mixing to completely dissolve and obtain sodium hydroxide-ethanol mixed liquid (A), adding the modified activated carbon carrier into the sodium hydroxide-ethanol mixed liquid (A) at 100 g/L, controlling the temperature at 40 degrees C, mixing at 300 rpm and soaking for 8 hours, filtering, washing the solid with absolute ethyl alcohol, and then filtering to obtain solid material (S). Step (3) is performed by adding anhydrous magnesium chloride into absolute ethyl alcohol at 100 g/L, mixing to completely dissolve and obtain magnesium chloride-ethanol mixed liquid (B), soaking the solid material (S) in the magnesium chloride-ethanol mixed liquid (B) at 20 g/L, controlling the temperature at 40 degrees C, mixing at 300 rpm and soaking for 8 hours, vacuum drying the solid material to a constant weight at 60 degrees C, calcining at 400 degrees C for 4 hours under nitrogen protection, calcining at 530 degrees C for 1 hour under nitrogen protection, and naturally cooling to room temperature to obtain the hydrophobic solid base catalyst (I) comprising magnesium oxide (MgO) and modified activated carbon carrier. An INDEPENDENT CLAIM is included for synthesis of biodiesel using grease containing water and free fatty acid as raw material, comprising (i) adding the grease, methanol and catalyst (I) into a reflux container, stirring at 400 rpm, reacting at 50-60 degrees C for 60-90 minutes under normal pressure; and (ii) filtering the reaction mixture to remove the catalyst and obtain biodiesel crude product, washing with water for not less than 3 times, removing the water phase, and continuously distilling the oil phase at 60-250 degrees C under reduced pressure to obtain the biodiesel. In step (i), the mole ratio of the grease to methanol is 1:12; and the mass ratio of the catalyst (I) to grease is 5-10 %.