NOVELTY - A biodiesel preparing method involves mixing waste edible oil with mineral diesel oil and water in a mass ratio of 1:10:1, heating mixture at 60-90 degrees C, standing for 10-60 minutes, removing water phase, heating at 100-150 degrees C, filtering mixture, heating at 110-180 degrees C, and decompressing and drying resultant product. The mixture is added with carbon bisulfide or dimethyl disulfide, followed by treating mixture at 150-350 degrees C under hydrogen partial pressure of 1.50-4.00 MPa, performing hydrogenation demetallization, degumming, partial hydrogenation deoxidization reaction, filling catalyst.    USE - Method for preparing biodiesel.    ADVANTAGE - The method enables preparing biodiesel with high quality.    DETAILED DESCRIPTION - A biodiesel preparing method involves mixing waste edible oil with mineral diesel oil and water in a mass ratio of 1:10:1, heating mixture at 60-90 degrees C, standing for 10-60 minutes, removing water phase, heating at 100-150 degrees C, filtering mixture, heating at 110-180 degrees C, and decompressing and drying resultant product. The mixture is added with 0.01-0.5% of carbon bisulfide or dimethyl disulfide, followed by treating mixture at 150-350 degrees C under hydrogen partial pressure of 1.50-4.00 MPa, where the volume ratio of hydrogen to oil is 500:1-1200:1, performing hydrogenation demetallization, degumming, partial hydrogenation deoxidization reaction, filling catalyst, where the protective catalyst is aluminum oxide or silicon oxide carrier, and second catalyst is nickel oxide, molybdenum oxide or tungsten oxide, followed by performing hydrogenation on resultant product. The resultant product is treated at under hydrogen partial pressure of 4.0-12.0 MPa in a bed layer at 300-450 degrees C, airspeed of 0.3-2.0 h-1, supplying into hydrogenation reactor, performing hydrogenation modification, adding catalyst, where the catalyst comprises molybdenum oxide or tungsten oxide loaded with aluminium oxide or silicon oxide carrier, cooling resultant product, performing vapor-liquid separation under reduced-pressure, treating in heating furnace and fractionating crude product to obtain finished product.