Metamizole sodium was chosen as a representative of unstable analytes for investigation by discussing the effects of oxygen and solvent on its degradation reaction using the capillary electmphoresis technique. A possible degradation mechanism was deduced from the observed behavior and was confirmed by infra-red spectroscopic study. The degradation reaction could be inhibited obviously by methanol instead of water as the solvent of analyte. Under the optimized conditions： separation voltage of 20 kV, and 5 mmol/L disodium hydrogen phosphate and 5 mmol/L borax with 10% methanol（pH 9. 12） as the running buffer, the standard curve of metamizole sodium was linear in a range of 3.77- 74. 07 mg/L. A satisfactory result was achieved when the technique was used to detect metamizole sodium in tablet.
