Abstract： Objective： To investigate the changes of antioxidant active components of Rehmannia Glutinosa before and after processing by online HPLC- UV- DPPH method. Methods： Taking the mullein glycosides as control， four batches of Rehmannia Glutinosa from Hubei Province and two batches from Anhui Province were selected as samples with serial numbers of S01 to S06. By taking a mobile phase of acetonitrile- 0.1% acetic acid solution，sample introduction of 20 μL at 25℃ ， and the wavelength detected by HPLC being 334 nm as well as the wavelength by DPPH being 517nm， the antioxidant activity was analyzed. The HPLC-FT-MS method(capillary temperature of 320 ℃，2.5 kV，100 μA) was used for component analysis. Results： The antioxidation activity titer was calculated by using the area of mullein antidrome as the control through the equation Y=0.0006X4-0.0241X3+0.4041X2+1.299X+1.652. After processing，the activity of Batch S02 was significantly decreased， while the active components of Batch S03， S04 and S05 were significantly increased， with little difference between the remaining batches. After processing，one active substance was reduced，but five active substances were added. Conclusion： Through an analysis of antioxidant activity of Radix Rehmanniae before and after processing by online HPLC-UV-DPPH method，there are both the formation of new antioxidant active substances and the disappearance of the original antioxidant active substances.