NOVELTY - Chemically synthesizing biodiesel comprises adding fatty acid and methanol into a primary fixed bed reactor to react, then passing fatty acid methyl ester and non reacted fatty acid into a second level fixed bed reactor, removing methanol and water, adding coal tar oil into a side part of the rectifying tower, extracting heavy component oil, passing the heavy component oil into a fractionating tower to separate light and heavy components, passing the heavy component into a heavy component rectifying tower, and performing re-separation at 150-2000 degrees C to obtain the finished product.    USE - The method is useful for chemically synthesizing biodiesel (claimed).    ADVANTAGE - The method is capable of chemically synthesizing biodiesel with high quality, less energy consumption and controlled production temperature.    DETAILED DESCRIPTION - Chemically synthesizing biodiesel comprises adding fatty acid of 13750 kg/hours and methanol of 3500 kg/hours into a primary fixed bed reactor to react at 95 degrees C for producing excess methanol and water at reaction stage, passing the methanol in vapor phase into a rectifying tower to recycle for obtaining 99.9% distilled methanol, then passing fatty acid methyl ester and non reacted fatty acid in a liquid phase from the first-stage fixed bed reactor into a second stage fixed bed reactor by a scraper evaporator, removing methanol and water under vacuum to obtain the finished product, adding coal tar oil with heavy component as raw material into a side part of the rectifying tower, extracting heavy component oil using a three-tower process by a heat exchanger, passing the heavy component oil into a fractionating tower to carry out separation of light and heavy components, passing light component in gas phase into a light component rectifying tower to separate the light component from a top of the tower, condensing the light component by a condenser to obtain the product, passing the heavy component from a bottom of the primary fractionating tower into a heavy component rectifying tower using a pump, and performing re-separation at 150-2000 degrees C to obtain the finished product.