Data of 660 radio-soundings launched at 9 o'clock during June, July and August, 1971-1978 and data of weather radar and surface observations in the same period in Yongdeng Hail Suppression Test site have been analysed. The results show that different weather systems have different inversion layer features. The thickness, intensity of upper inversion layer and its altitude and corresponding temperature indicate the severity of hailstorms. The more the tropopause descends and the more the inversion layer is close to or breakthrough the tropopause, the more the severity of hailstorm is. The paper discusses the inversion layer feature of hailstorms and the role it plays. The could be helpful to forecast correctly hailstorms and provide basis for further revealing the background of hailstorms.