The invention discloses a preparation method of
levofloxacin-N-
oxide. In the existing preparation method, a large number of reaction impurities exist, purification cannot be carried out easily, the consumption of
hydrogen peroxide in reaction is large, and the subsequent treatment is troublesome. The preparation method comprises the following steps of: mixing
levofloxacin and a
solvent glacial
acetic acid, and stirring the mixture at 60-70 DEG C until
dissolution; adding catalyst which can be
phosphotungstic acid,
phosphomolybdic acid or
sodium tungstate; adding
hydrogen peroxide solution with the
mass concentration of 25-35 percent in batches, and making the mixture react for 2-4 hours after each batch is added, and adding the
hydrogen peroxide solution until the reaction is fully completed; carrying out
vacuum distillation on reaction solution prepared via the reaction until the volume of the solution is one third of the original volume, adding an appropriate amount of water, then carrying out
vacuum distillation on reaction solution again until the volume of the solution is one third of the original volume, repeating the
vacuum distillation twice to three times, and
drying via
evaporation; and carrying out vacuum
distillation on the prepared residues, adding a recrystallization
solvent for recrystallization to obtain the
levofloxacin-N-
oxide. The invention has the advantages of fewer side actions, higher product purity, low consumption of the
hydrogen peroxide, and convenient subsequent treatment.