(S)-10-(1-carbamoylcyclopropyl)-9-fluoro-3-methyl-7-oxo-3,7-dihydro-2H -[1,4]-oxazino[2,3,4-ij]quinoline-6-carboxylic acid, the senior intermediate of pazu- floxacin, was synthesiszed from (S)-ethyl 10-(cyanomethyl)-9-fluoro-3-methyl- 7-oxo-3,7-dihydro-2H-[1,4]oxazino[2,3,4-ij]quinoline-6-carboxylate, via a one-pot reaction of cyclopropanation and cyanide hydrolyzation. The reaction conditions were optimized as follows: n (Compound II): n(1,2-dichloroethane): n (Benzyltriethylam monium chloride):n(NaOH) = 1:1.5:0.43:16.7，stirred in water for 5h at room temperature and then reflux for 2h. The method has the advantages of mild reaction condition, safety, low cost, and environmental friendly.