NOVELTY - Producing biodiesel by one-step method acid catalysis using waste oil comprises: (a) hydrolyzing the waste oil at normal pressure to obtain black fatty acid; (b) vacuum dehydrating black fatty acid; (c) performing esterification and dealcoholization; (d) neutralizing it and washing it with water; (e) dehydrating rough methyl ester; and (f) distilling rough methyl ester.    USE - The process is useful for producing biodiesel by one-step method acid catalysis using waste oil (claimed).    ADVANTAGE - In utilizing the process, the product fatty acid methyl ester has a purity of more than 98% and has stable and reliable quality; where it satisfies the demand of the biodiesel for the internal-combustion engine.    DETAILED DESCRIPTION - Producing biodiesel by one-step method acid catalysis using waste oil comprises: (a) hydrolyzing the waste oil at normal pressure to obtain black fatty acid: acidifying the waste oil by adding water and concentrated sulfuric acid; where the quantity of water and the quantity of concentrated sulfuric acid is respectively 10-100% and 3-10% of the weight of the waste oil; increasing temperature to 100 degrees C, adding effective catalyst having a quantity of 1.0-5.0% of the weight of the waste oil to hydrolyze it at normal temperature, and hydrolyzing it for 2-3 hours; where when the acid value is more than 180 mg potassium hydroxide (KOH)/g, the black fatty acid is obtained; (b) vacuum dehydrating black fatty acid: dehydrating black fatty acid at condition of vacuum, controlling the temperature for dehydrating at 100-130 degrees C and the vacuum pressure at 0-4 KPa; (c) performing esterification and dealcoholization: adding the dehydrated black fatty acid into the reaction kettle, adding methanol and concentrated sulfuric acid; where the quantity of the methanol and the quantity of the concentrated sulfuric acid is respectively 20-50% and 0.5-2.0% of the weight of the black fatty acid; performing esterification for 2.0-4.0 hours, distilling it and removing the exceeded methanol, recycling and reusing the steamed methanol; (d) neutralizing it and washing it with water: cooling it after performing esterification and dealcoholization, neutralizing sulfuric acid with 10% of sodium carbonate diluted solution to make the pH be about 7.0; where the quantity of the sodium carbonate is 1.1 times of the quantity of the sulfuric acid; standing it and discharging the wastewater at the bottom part to obtain the rough methyl ester, washing the rough methyl ester with hot water having a temperature of 60-80 degrees C; where the quantity of the hot water is 20% of the quantity of the rough methyl ester; and washing it with water for 1-2 times; (e) dehydrating rough methyl ester: dehydrating the washed rough methyl ester at 100-130 degrees C and at vacuum pressure of 0-4 KPa; and (f) distilling rough methyl ester: adding the dehydrated rough methyl ester into the distilling pot, sending the conductive oil to the liner of the distilling pot, heating the rough methyl ester, controlling the temperature in the distilling pot at 225-260 degrees C and a vacuum pressure of 0-2 KPa, and decompressing and distilling it to obtain the biodiesel.