NOVELTY - Preparation of biodiesel by waste oil split-phase process, involves (a) heating waste oil to 80 degrees C, treating oil with sulfuric acid and 85 degrees C hot salt water to obtain degummed waste oil; (b) adding methanol to the degummed oil, standing and allowing separation of mixture to glyceride phase and fatty acid phase; (c) adding methanol and caustic soda to glyceride phase, standing and centrifugally separating the layer to obtain crude biodiesel, washing and separating the crude biodiesel to obtain biodiesel; and (d) esterifying the fatty acid phase to obtain biodiesel.    USE - The method is useful for preparing biodiesel from waste oil.    DETAILED DESCRIPTION - Method for preparing biodiesel by waste oil split-phase process, involves (a) heating waste oil to 80 degrees C, adding 98% concentrated sulfuric acid to 0.3% oil, stirring for 20 minutes, adding 2% of 85 degrees C hot salt water to 5% oil, stirring the mixture for 30 minutes, and separating the mixture to obtain degummed waste oil; (b) adding methanol to the degummed waste oil (1.5:1), stirring the mixture for 30 minutes at 30 degrees C, and then stirring for 30 minutes, standing the mixture and allowing separation of mixture to lower glyceride phase and upper fatty acid phase, sampling and measuring acid value of glyceride, repeating the measurement until acid value is lowered to 2-3, and then separating the fatty acid phase and glyceride phase; (c) adding methanol to the glyceride phase, where the ratio of glyceride to methanol is 1:4.5, adding solid caustic soda (0.8 wt.% of the amount of glyceride) to glyceride, stirring and blending mixture, reacting the mixture for 1 hour at 70 degrees C, standing and separating the glycerin layer from the reaction mixture, neutralizing the mixture immediately using concentrated sulfuric acid, and centrifugally separating the neutralized mixture to obtain the crude biodiesel, crude glycerol and sodium sulfate, adding 80 degrees C water solution and washing the crude biodiesel, centrifugally separating the mixture and separating water layer, evaporating methanol from the biodiesel until methanol is eliminated completely, cooling and filtering biodiesel, washing the filtrate with water and drying to obtain biodiesel; and (d) adding methanol to the fatty acid, adding sulfuric acid for catalysis, where the weight ratio of fatty acid to methanol to sulfuric acid is 1:0.45:0.03, performing esterification at 60-65 degrees C stage by stage for 30 minutes respectively, performing vacuum dealcoholization and dewatering between the two stages of reaction, and finally cooling and filtering to obtain biodiesel.