Lagotis brevituba Maxim is a perennial species distributed in the highlands of China, which has been used for more than 2000 years as a traditional Tibetan medicinal plant. However, no attention has been paid to the antioxidant activities of Lagotis brevituba Maxim in vitro or in vivo. Thus, this study aimed to evaluate the in vitro and in vivo antioxidant activity of Lagotis brevituba Maxim against radiation‐induced damage as well as the systematic chemical components. To explore the relationship between the antioxidant activity and extraction solvent, Lagotis brevituba Maxim was extracted with three different solvents: methanol, water, and acetone. In antioxidant assays in vitro, the water extract had the strongest reducing power, 2,2‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid) diammonium salt (ABTS) radical, and 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) radical scavenging activity compared with the methanol and acetone extracts. However, the methanol extract was more potent in the β‐carotene/linoleic acid cooxidation assay. In antioxidant assays in vivo, mice that were exposed to 6.0 Gy⁶⁰Co γ‐ray whole‐body radiation on day 15 after administration of Lagotis brevituba Maxim decreased their level of malondialdehyde (MDA) in a dose‐dependent manner compared with the control group, indicating that Lagotis brevituba Maxim had favorable antioxidant activities in vivo. In addition, a total of 44 compounds were tentatively identified by liquid chromatography electrospray ionization quadrupole time‐of‐flight mass spectrometry (LC‐ESI‐QTOF‐MS/MS), including 19 flavonoids, 14 phenols, 8 phenylethanoid glycosides, 2 iridoid glycosides, and 1 carbohydrate. We obtained 25 compounds from plants in the genus Lagotis for the first time. These results suggested that Lagotis brevituba Maxim had potent antioxidant activity and could be explored as a novel natural antioxidant.