Abstract: The scavenging activities of Ophiopogon japonicus for superoxide anion (O2^·-) radicals by HPLC-DAD coupled with chemiluminescence(CL) was studied and the active constituents in it were screened.The aqueous extracts of Ophiopogon japonicus were divided into the ethyl ether fraction and the n-butanol fraction.Their activities were investigated respectively and the reduction of CL with the pyrogallic acid 3amino-phthalhydrazid (luminol)-Na₂CO₃/NaHCO₃ buffer (pH 11.0) system worked as an activity index of the antioxidant constituents.Major active components in Ophiopogon japonicus could be identified by HPLC-DAD-MS/MS and the reference substances.The antioxidant activities of different ingredients were evaluated by standard potency equations using quercetin as positive control.The experimental data indicated that the scavenging activities of Ophiopogon japonicus for superoxide anion (O2^·-) radicals are mainly from homoisoflavone in the ethyl ether fraction.It has shown that the method for on-line detection is an advantageous tool to rapidly determinate antioxidants from complex herbs.