Abstract: Radiation brain injury is a serious complication among the radiotherapy of brain tumors. It is demonstrated that the protective action of magnesium ion in the brain injury from some experimental studies recent years, which is the prospective neuroprotective agents overall merits.This article is summarized the causes and the variance of magnesium ion in the brain tissue afterwards the radioactive brain injury, additionally the defense mechanism of magnesium ion from the aspects of inflammation reduction, encephaledema alleviation, anti-apoptosis and improcement of nerve function.