NOVELTY - Biodiesel is prepared by preprocessing fat rich in free fatty acid, filtering and centrifuging to remove impurities and dehydrate; mixing fat rich in free fatty acid with methanol, adding solid acid catalyst, reacting at 60-80 degrees C for 2-5 hours to esterify fatty acid in fat, centrifuging to remove catalyst, standing and collecting oil phase; and adding methanol and solid alkali catalyst to oil phase, reacting at 60-80 degrees C for 1.5-2.5 hours to esterify triglyceride into fatty acid methyl ester, removing catalyst, standing, collecting fatty acid methyl ester phase and steaming to remove methanol.    USE - Method of preparing biodiesel (claimed).    ADVANTAGE - The method has low cost, does not discharge wastewater during the whole process such that the environment is not polluted, and uses solid catalyst that are inorganic compound, where the catalyst has low cost and has high catalytic efficiency.    DETAILED DESCRIPTION - Biodiesel is prepared by preprocessing fat rich in free fatty acid, filtering and centrifuging to remove impurities and dehydrate; mixing fat rich in free fatty acid with methanol at a mol ratio of 1:9 to 1:20, adding solid acid catalyst (1.5-4 wt.%), reacting at 60-80 degrees C for 2-5 hours to esterify fatty acid in fat, centrifuging to remove catalyst, standing, and collecting oil phase; and adding methanol and solid alkali catalyst (1-5 wt.%) to oil phase, where the mol ratio of oil phase to methanol is 1:6 to 1:12, reacting at 60-80 degrees C for 1.5-2.5 hours to esterify triglyceride into fatty acid methyl ester, removing catalyst, standing, collecting fatty acid methyl ester phase, and steaming to remove methanol. The fat rich in free fatty acid is waste meal oil, oil residue from oil extraction and/or soapstock acidification oil. The solid acid catalyst is zirconium sulfate tetrahydrate, ferric sulfate, or load type solid acid catalyst prepared with zirconium sulfate tetrahydrate or ferric sulfate. The load type solid acid catalyst is prepared by dissolving (pts. wt.) zirconium sulfate or ferric sulfate (30-40) in distilled water (100), adding carriers (100), stirring at 60 degrees C for 1.5-2 hours, then drying at 100 degrees C, and roasting at 500-600 degrees C for 4-6 hours. The carriers include alumina, kieselguhr, silica gel and molecular sieves.