NOVELTY - Preparing entecavir (I), comprises: adding tetrahydrofuran (II) and zinc under a nitrogen atmosphere, reducing the temperature to -40 degrees C, then adding dibromomethane, stirring and reacting the mixture in the presence of titanium tetrachloride to obtain a Nysted reagent (III); and adding (2R,3S,5S)-3-benzyloxy-5-(6-benzyloxy-2-(((4-methoxyphenyl)-diphenyl-methyl)amino)purin-9-yl)-2-(benzyloxymethyl)cyclopentanone (IV) and anhydrous chloroform into a reactor under the protection of nitrogen, and then adding lithium, 4-dimethylaminopyridine and triethanolamine, and reacting the mixture.    USE - The method is useful for preparing (I) (claimed) that is useful for treating hepatitis B.    ADVANTAGE - (I) can be prepared in a simple and eco-friendly manner with high yield and purity.    DETAILED DESCRIPTION - Preparing entecavir of formula (I), comprises: (a) adding tetrahydrofuran of formula (II) and zinc under a nitrogen atmosphere, reducing the temperature to -40 degrees C, then adding dibromomethane, stirring the mixture at 5 degrees C, and reacting the mixture in the presence of titanium tetrachloride to obtain a Nysted reagent of formula (III); (b) adding (2R,3S,5S)-3-benzyloxy-5-(6-benzyloxy-2-(((4-methoxyphenyl)-diphenyl-methyl)amino)purin-9-yl)-2-(benzyloxymethyl)cyclopentanone of formula (IV) and anhydrous chloroform into a reactor under the protection of nitrogen, then adding lithium, 4-dimethylaminopyridine and triethanolamine, stirring the mixture, reacting the mixture at 20 degrees C for 24 hours in the presence of 1-(chloro(diphenyl)methyl)-4-methoxy-benzene of formula (V), and performing a silica gel column chromatography to obtain enantiomer-8 of formula (VI); (c) adding dichloromethane and (VI) into a reaction tank under nitrogen atmosphere, then adding (III), stirring the mixture at room temperature for 5 hours, mixing the reaction liquid with a solution of saturated sodium bicarbonate and chloroform, stirring for 30 minutes, centrifuging, separating, extracting using chloroform for three times, and performing column chromatography to obtain enantiomer-9 of formula (VII); (d) adding tetrahydrofuran, methanol and (VII) into a reaction bottle, stirring, adding hydrochloric acid, heating at 50 degrees C, cooling to room temperature after the reaction, adding sodium hydroxide to adjust a pH value of 7, extracting using ethyl acetate for 3 times, filtering, rotary steaming to remove the solvent to obtain residue, and using diethyl ether to obtain enantiomer-10 of formula (VIII); and (e) adding (VIII) and dichloromethane into the reaction tank under a nitrogen atmosphere, stirring and reducing the temperature to 80 degrees C, adding a solution of boron trichloride and dichloromethane, increasing the temperature to 40 degrees C, then reacting for 3 hours, adding methanol, rotary steaming to remove the solvent to obtain entecavir crude product, and refining to obtain (I).