NOVELTY - Waste oil and methanol as raw materials are mixed, catalyzed, transesterified using continuous flow microchannel reactor and external heat exchanger, and separated using metered pump connected to each module, to obtain biodiesel product. Waste oil and methanol are mixed in series-enhanced microchannel module and direct-current microchannel module, esterified, and product is discharged from outlet of reactor and cooled, to obtain crude biodiesel. Crude biodiesel is mixed with acid or alkali catalyst, filtered to remove impurities and distilled, to obtain biodiesel.    USE - Production of biodiesel (claimed).    ADVANTAGE - The method enables production of biodiesel with high yield by simple, environmentally-friendly and safe method.    DETAILED DESCRIPTION - Waste oil and methanol as raw materials are mixed, catalyzed, transesterified using continuous flow microchannel reactor and external heat exchanger, and separated using metered pump connected to each module, to obtain biodiesel product. The temperature of heat exchanger is set and controlled. The heat exchanger medium is hot oil. The molar ratio of waste oil and methanol is 1:4-1:10. The flow rate of waste oil is controlled to 1-5 mL/minute. The flow rate of methanol is controlled to 10-20 mL/minute. Waste oil and methanol are pumped via respective metering pumps and subjected to mass transfer within modules, mixed ad reacted. The mixing temperature is controlled by external heat exchanger. The flow rate and molar ratio of waste oil and methanol are controlled, mixed in series-enhanced microchannel module and direct-current microchannel module, esterified at 50-80 degrees C and 0.11.5 MPa, and product is discharged from outlet of reactor and cooled. The residence time in microchannel reactor is 50-100 seconds. The reaction solution from outlet of microchannel reactor is cooled, to obtain crude biodiesel. Crude biodiesel is mixed with acid or alkali catalyst, filtered to remove impurities and distilled, to obtain biodiesel with yield of 95-98%.