0515 Typhoon“Khanun”caused serious destruction in Zhejiang Province. The reasons why the outer wind speed of the typhoon is much spedier than the near central wind speed have been analyzed in this article. The results show that the strengthening subtropical high pressure, the momentum transmission of jet stream in the middle-lower troposphere, a violent downdraft from upper air and a sudden intensification of the outer pressure gradient force of the tyhoon are the important factors due to the strong outer wind of typhoon.