NOVELTY - The method comprises adding green tea and extracting solution in a homogenate machine for processing homogenate, adjusting the pH value of sulfuric acid water solution with concentration of 0.5-5% to 2-3.5%, mixing the extracting solution with the sulfuric acid water solution and inorganic salt water solution, where the pH value of the mixed solution is 2-3.5 and the molar concentration of the inorganic salt is 1-3 M, processing the homogenate via an ultrasonic extracting machine with a frequency of 20 KHz to 1 MHz, and removing the solid with a sedimentation centrifuge.    USE - The method is useful for extracting caffeine from green tea.    ADVANTAGE - The method is capable of rapidly and simply extracting the caffeine from green tea with high efficiency and without pollution.    DETAILED DESCRIPTION - The method comprises adding green tea and extracting solution in a homogenate machine for processing homogenate, adjusting the pH value of sulfuric acid water solution with concentration of 0.5-5% to 2-3.5%, mixing the extracting solution with the sulfuric acid water solution and inorganic salt water solution, where the pH value of the mixed solution is 2-3.5 and the molar concentration of the inorganic salt is 1-3 M, processing the homogenate via an ultrasonic extracting machine with a frequency of 20 KHz to 1 MHz, removing the solid with a sedimentation centrifuge to obtain the extract liquid, where the rotating speed is 1000-6000 r/min and the centrifugation time is 3-20 minutes, adjusting the pH value of the extract liquid to 8-11 with strong ammonia water, adding the extract liquid with organic solvent such as ethyl acetate, concentrating the extract liquid to obtain the mixture of caffeine at a temperature of 35-50 degrees C and a negative pressure of 0.07-0.1 Mpa or at normal pressure, purifying the mixture of caffeine with normal phase silica gel column, eluting with the eluent, where trichloromethane solution containing 3-10% of carbinol is used as eluent, merging the eluent containing caffeine, recovering the solvent at 35-50 degrees C and 0.06-0.1Mpa to obtain a crude product of caffeine, and processing recrystallization of the crude product of caffeine to obtain the end product of caffeine. The solvent used for recrystallization is 85-95% of ethanol solution.