NOVELTY - Preparing 3-methyl-4,4,7-trioxo-3-(1,2,3)triazol-1-ylmethyl-4lambda6-thia-1-aza-bicyclo(3.2.0)heptane-2-carboxylic acid benzhydryl ester (V) comprises preparing compound (II) from 3,3-dimethyl-4,7-dioxo-4lambdaasterisk4asterisk-thia-1-aza-bicyclo(3.2.0)heptane-2-carboxylic acid benzhydryl ester (I), then preparing compound (III) and 3-methyl-4,4,7-trioxo-3-(1,2,3)triazol-1-ylmethyl-4lambdaasterisk6asterisk-thia-1-aza-bicyclo(3.2.0)heptane-2-carboxylic acid benzhydryl ester (IV), and reacting (IV) with potassium permanganate and sulfuric acid in presence of dichloromethane, acetic acid and water.    USE - The method is useful for preparing (V).    ADVANTAGE - The method: is safe and environment-friendly; uses a resin, which can be recycled and improves the reaction yield (higher than 75%); reduces the dosage amount of glacial acetic acid and produces products with stable quality; has mild reaction conditions and can be controlled easily; avoids azide reaction and ethynylation reaction; and is economical and does not require any special equipment, thus it is suitable for industrial production.    DETAILED DESCRIPTION - Preparing 3-methyl-4,4,7-trioxo-3-(1,2,3)triazol-1-ylmethyl-4lambda6-thia-1-aza-bicyclo(3.2.0)heptane-2-carboxylic acid benzhydryl ester (V) comprises (i) reacting 3,3-dimethyl-4,7-dioxo-4lambdaasterisk4asterisk-thia-1-aza-bicyclo(3.2.0)heptane-2-carboxylic acid benzhydryl ester (I) with benzo(d)thiazole-2-thiol to obtain 2-(2-(benzothiazol-2-yldisulfanyl)-4-oxo-azetidin-1-yl)-3-methyl-but-3-enoic acid benzhydryl ester (II), (ii) reacting (II) with cupric bromide or hydrogen bromide and sodium nitrite to obtain 3-bromomethyl-3-methyl-4,7-dioxo-4lambdaasterisk4asterisk-thia-1-aza-bicyclo(3.2.0)heptane-2-carboxylic acid benzhydryl ester (III), (iii) reacting (III) with 2H-1,2,3-triazole to obtain 3-methyl-4,4,7-trioxo-3-(1,2,3)triazol-1-ylmethyl-4lambdaasterisk6asterisk-thia-1-aza-bicyclo(3.2.0)heptane-2-carboxylic acid benzhydryl ester (IV), and (iv) reacting (IV) with potassium permanganate and sulfuric acid in presence of dichloromethane, acetic acid and water.