The present invention relates to an extract of Petasites japonicus having antioxidant activity, and more particularly, to an extract of Petasites japonicus having antioxidant activity which is extracted using an alcohol and an antioxidant for the prevention and treatment of a brain disease comprising the extract as an effective ingredient. The Petasites japonicus extract according to the present invention has excellent antioxidant activity, relief of neurotoxicity and neuroprotective effect and thus, can be usefully used in the prevention and treatment of brain diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, hypoxic-ischemic injury and chronic spinal cord injury.