Facts about precipitation change in last hundred years. Precipitation data in Changsha shows a trend of increment in the last hundred years and inclination rate is 97mm per hundred years. The maximum in precipitation of each decade, in days for torrential rain, in precipitation of a day are all occurred at 1990's. The days for torrential rain is 1.5 times in 1990's than the average decade and the maximum precipitation per day in 1990's is by far the average decade. Extreme climate incident always do extreme damage to economy. Change of precipitation and that of floods area is synonymous. As for floods, the 1990's were the most frequent and intense decade in Hunan province, every year experienced the floods excess the medium in the scope. After 1949, there were 9 years in which catastrophic floods had occur in total, among them 7 were belong to 90's.The agriculture losses resulted from floods are great in Hunan Province. Since 1949, there were 358.72×10³ ha stricken area per year on the average, among them 250×10³ ha were afflicted area. The floods were getting frequent and condition of floods were worsened in 1990's and there were 800.22×10³ ha stricken area per year on the average, among them 403.332×10³ ha were afflicted area. Dongting Lake catchment is a radiation shaped river net with Dongting Lake located in center and branches converged from all direction. Its floods comes from four mouths of Yangtze and Xiang, Zi, Yuan, Li four rivers within Hunan boundary as well. Not only the chance for floods of four rivers meet together is great but also floods of four rivers and Yangtze meet together is great, especially floods of Yuan, Li river meet with that of Yangtze. The center for heavy rain wander within 100×10⁴ km² and reflected as flooding of each branch by turns in upper river and piling up of flood peaks when flowing into Dongting Lake. The Yangtze River's high peaked, tremendous, lasting floods can cause catastrophic disaster when meets floods of four rivers. We assess the ability for sustaining disaster and the risk confronted in hundred counties of Hunan, utilize the GIS software to draft a county based distribution map about the ability of sustenance disaster in Hunan Province, Finally, a proposal on the corresponding countermeasures on the disaster mitigation is put forward.