NOVELTY - Fuel ethanol is produced by drying fresh duckweed at 60 degrees C, crushing, screening using 60-mesh sieve, mixing dried duckweed powder and water to obtain duckweed fermentation culture medium, subjecting the culture medium to an ultrasonic generator, pretreating, and cooling the medium using and ice water bath; and heating the culture medium at 95 degrees C for 5 minutes, adding alpha -amylase to liquefy, compensating the vaporization water, sterilizing at 115 degrees C for 20 minutes, and cooling to 50 degrees C.    USE - Method of producing fuel ethanol (claimed).    ADVANTAGE - The method is simple, short and low cost; does not produce pollutants; and has high efficiency.    DETAILED DESCRIPTION - Fuel ethanol is produced by drying fresh duckweed at 60 degrees C, crushing, screening using 60-mesh sieve, mixing dried duckweed powder and water to obtain duckweed fermentation culture medium, and subjecting the culture medium to an ultrasonic generator controlled at a frequency of 22 kHz and power of 400-700 W for 100-200 seconds; cooling the medium using and ice water bath, heating the culture medium at 95 degrees C for 5 minutes, adding alpha -amylase (100-200, preferably 150-200 mu l/100 g of medium) for liquefaction to obtain reddish brown material, compensating the vaporization water, sterilizing at 115 degrees C for 20 minutes, and cooling to 50 degrees C; and adding pre-treatment enzyme (100-200, preferably 150-200 mu l/100 g of medium) and saccharifying enzyme under aseptic condition, pretreating and saccharifying at 50 degrees C while stirring at a speed of 180-220 revolutions/minute for 2 hours, inoculating (ml) Saccharomyces cerevisiae seed liquid (10) and mixed vitamin liquid (1) to every 100 g of duckweed culture medium, or cooling the duckweed culture medium at 30 degrees C, adding pre-treatment enzyme and saccharifying enzyme under aseptic condition, inoculating S. cerevisiae seed liquid and mixed vitamin liquid, stirring at 180-220 revolutions/minute, pre-treating, saccharifying and fermenting synchronously.