NOVELTY - Production of biodiesel oil includes mixing fatty acid having acid number of 190-210 mg KOH/g and heated to 50-60 degrees C, anhydrous methyl ester and sulfuric acid catalyst at weight ratio of 80-85:10-20:1-2; preesterifying; finely esterifying; washing, desulfurizing and esterifying to obtain crude methyl ester; distilling crude methyl ester at 130-140 degrees C and separating refined methyl ester; depressing freezing point at low temperature; recycling waste gas; and treating waste liquid.    USE - Method for producing biodiesel oil (claimed).    ADVANTAGE - The method is environment friendly, saves energy by using low-temperature heat source, and can realize mass continuous production. Esterification time is shortened to 1 hour, esterification rate reaches to 98%, and yield is increased because the processes are rational and compact. The biodiesel oil has low freezing point because wax in the oil is removed efficiently. The produced waste gas and liquid can be reused after separation and purification to reduce discharge capacity.    DETAILED DESCRIPTION - Production of biodiesel oil includes:    (A) mixing fatty acid having acid number of 190-210 mg KOH/g and heated to 50-60 degrees C, anhydrous methyl ester and sulfuric acid catalyst (greater than 98 wt.%) at weight ratio of 80-85:10-20:1-2, in which the fatty acid is obtained from vegetable or animal leftovers;    (B) reacting the mixture at 60-70 degrees C, less than or equal to 4 MPa and 60-100 rpm for 30 minutes, separating, and removing upper layer material with acid number of 60-70 mg KOH/g;    (C) treating the upper layer material to obtain material with acid number of 3-6 mg KOH/g;    (D) adding methanol (10-20 wt.%), mixing, esterifying at 60-70 degrees C and less than or equal to 4 MPa, separating, esterifying upper material, and separating to obtain crude methyl ester, in which liquid hourly space velocity for esterifying is 30 minutes;    (E) distilling crude methyl ester at 130-140 degrees C, condensing methanol gas and steam and using them as esterifying and washing agent, and separating refined methyl ester with acid number of 0.3-0.7 mg KOH/g and freezing point of 5-15 degrees C;    (F) sending refined methyl ester to a jacketed crystallizer, crystallizing wax, and removing crystals by automatic squeezing filter to eliminate wax (3-6%) and form ester with freezing point of - 5 to - 25 degrees C;    (G) collecting waste gas, cooling gas into liquid, purifying by rectification, and reusing methanol; and    (H) sending lower liquids formed by preesterifying and finely esterifying to a liquid separator, washing waste oil on upper layer with methanol (10-30 wt.%), distilling to remove methanol, using the oil as boiler fuel, sending the mixture of sulfuric acid, methanol and esterifying water in lower layer to a scraper distiller, distilling at 130-140 degrees C, purifying methanol gas and steam, reusing methanol, and using sulfuric acid as catalyst.