Objective: To study the scavenging activities of Xiaoaiping injection for ABTS·+ by HPLC to trace the active constituents on-line. Method: Solution of Xiaoaiping injection was passed through HPLC to enter into the PEEK coil contained the ABTS·+ working solution

and signals were recorded

and their chemical constituents were identified by standard references

and their scavenging rates were calculated. Result: Neochlorogenic acid

caffeic acid and chlorogenic acid were identified

about the scavenging activities for ABTS·+

and their scavenging rates were 9.9%

22.5%

16.5% respectively. Conclusion: Phenolic acids in Xiaoaiping injection make a significant contribution to the scavenging activities for ABTS·+. It is shown that the method for on-line detection is an advantageous tool to rapidly determine antioxidants from traditional Chinese medicine.