Based on the meteorological data collected from 1084 stations over North China, Central China and East China from 1971 to 2000, features of interdecadal variation of thunderstorm, hail and gale wind in the south of the Yangtze River (31°N southward), Jiang Huai area and Huang Huai (31°N to 37°N) and the north of the Yellow River (37°N northward) have been analyzed. The analysis results have indicated that the frequency of thunderstorm, hail and gale wind has a decreasing trend. The inter decadal variation of thunderstorm in the south of the Yangtze River shows a more significant decreasing trend, hail decreases most obviously in the north of the Yellow River, and gale wind decreases evidently in all these three regions. The hail often occurs with gale wind, and more frequent in the plateau and the mountain region than in the plain. It takes place most in northern North China and Central Inner Mongolia. The analysis of relevant conditions for the generation and development of the mesoscale convective system such as water vapor, thermodynamic instability and dynamical lifting has shown that the reduction of convection weather has a close relation to the decline of moisture and dynamic conditions. The composite index SWEAT (severe weather threat) including thermodynamic, dynamic and moisture conditions expresses the similar characteristics to the convection weather in the spatial distribution or the evolution trend in 30 years.