NOVELTY - Preparing entecavir (I) and its tautomer (Ia), comprises carrying out reduction reaction of (1S,3S,5S)-3-(2-amino-6-oxo-1H-purin-9(6H)-yl)-5-hydroxy-2-methylenecyclopentanecarboxylate compounds (II) or their tautomers (IIa) in an organic solvent under action of a reducing agent, where the reducing agent is diisobutylaluminum hydride, lithium aluminum hydride, Red-Al (RTM: Sodium bis(2-methoxyethoxy)aluminum hydride) or sodium borohydride, and the reaction is carried out in presence of lithium chloride and/or lithium bromide when the reducing agent is sodium borohydride.    USE - The method is useful for preparing entecavir and its tautomer (claimed).    ADVANTAGE - The method: utilizes cheap and easily available raw materials; has mild reaction conditions, less side effects, and high product yield; and is environment-friendly, and suitable for industrial production.    DETAILED DESCRIPTION - Preparing entecavir of formula (I) and its tautomer of formula (Ia), comprises carrying out reduction reaction of (1S,3S,5S)-3-(2-amino-6-oxo-1H-purin-9(6H)-yl)-5-hydroxy-2-methylenecyclopentanecarboxylate compounds of formula (II) or their tautomers of formula (IIa) in an organic solvent under action of a reducing agent, where the reducing agent is diisobutylaluminum hydride, lithium aluminum hydride, Red-Al (RTM: Sodium bis(2-methoxyethoxy)aluminum hydride) or sodium borohydride, and the reaction is carried out in presence of lithium chloride and/or lithium bromide when the reducing agent is sodium borohydride. An INDEPENDENT CLAIM is also included for (1R,3S)-3-(2-amino-6-oxo-1H-purin-9(6H)-yl)-2-methylenecyclopentanecarboxylate compounds of formula (III) or their tautomers of formula (IIIa), and (II) or (IIa).    R = 1-4C alkyl;    R1 = cyclic silane group of formula (IV) or (IVa), preferably 2-hydroxy-2,3-dihydro-1H-benzo(c)silol-2-yl, (3aS,7aS)-2-hydroxyoctahydro-1H-benzo(c)silol-2-yl, 1-hydroxysiletanyl, 1-hydroxysilepanyl, 3-hydroxy-1,5,3-dioxasilepan-3-yl, 1-hydroxysilolanyl, 4-hydroxy-1,4-oxasilinan-4-yl, 1-hydroxy-2,5-dimethylsilolanyl, 1-hydroxysilinanyl, 4-hydroxy-1-methyl-1,4-azasilinan-4-yl, (3R,4R)-1-hydroxy-3,4-dimethylcyclopentyl or (3S,4S)-1-hydroxy-3,4-dimethylcyclopentyl; either    Y1 = 3-9C hydrocarbon moiety (optionally substituted), which is connected with silicon atom to form 4-10 membered saturated or unsaturated cyclic silane (optionally substituted with 1-3C alkyl); or    Y1 = hetero atom-containing 3-9C hydrocarbon moiety (optionally substituted), which is connected with silicon atom to form 4-10 membered saturated or unsaturated heterocyclic silane (optionally substituted with 1-3C alkyl) containing 1-3 O, S and N atoms;    n, p = 0-2;    q = 1 or 2;    X, Y2 = C, O or N; and    R4 = 1-3C alkyl.