NOVELTY - Preparing flucloxacillin sodium comprises (i) adding 3-(2-chloro-6-fluorophenyl)-5-methylisoxazole-4-carboxylic acid into methylene chloride at -10-10 degrees C, dropping triethylamine, reacting, dropping pivaloyl chloride and reacting to obtain mixed anhydride, (ii) adding 6-aminopenicillanic acid into methylene chloride, adding triethylamine, reacting and clarifying to obtain 6-aminopenicillanic acid triethylamine salt solution, (iii) adding mixed anhydride into the 6-aminopenicillanic acid triethylamine salt solution, reacting and vacuum distilling, and (iv) crystallizing.    USE - The method is useful for preparing flucloxacillin sodium (claimed).    ADVANTAGE - The method is simple and ensures mild reaction conditions, produces high yield and product with good purity, and is environmentally friendly.    DETAILED DESCRIPTION - Preparing flucloxacillin sodium comprises (i) adding 3-(2-chloro-6-fluorophenyl)-5-methylisoxazole-4-carboxylic acid into methylene chloride at -10-10 degrees C, dropping triethylamine, reacting to form clear liquid, dropping pivaloyl chloride and reacting for 0.5-2 hours to obtain mixed anhydride, where the molar ratio of 3-(2-chloro-6-fluorophenyl)-5-methylisoxazole-4-carboxylic acid, triethylamine and pivaloyl chloride is 1:(1-2):(1-1.5), (ii) adding 6-aminopenicillanic acid into methylene chloride, adding triethylamine, reacting at 10-40 degrees C and clarifying to obtain 6-aminopenicillanic acid triethylamine salt solution, where the molar ratio of 6-aminopenicillanic and triethylamine is 1:(1-3), (iii) adding mixed anhydride into the 6-aminopenicillanic acid triethylamine salt solution, reacting at -10 to 10 degrees C for 0.5-2 hours, and vacuum distilling methylene chloride to obtain oily substance, adding acetone, stirring and filtering to remove triethylamine hydrochloride to obtain acetone solution of flucloxacillin triethylamine salt, and (iv) adding water and ethyl acetate solution of sodium iso-octoate, and crystallizing at 0-40 degrees C for 1-5 hours to obtain flucloxacillin sodium monohydrate.