Based on the sounding data at Huaihua and Changsha weather stations, Hunan Province from 1994 to 2003, the cloud water vapor content in the vertical atmosphere columniation are analyzed and computed over Zhexi reservoir and drainage area of Hunan province. It shows that the annual average value of cloud water vapor content is 3.62g·cm^-2 and varies with the synoptic pattern in Zhexi reservoir and drainage area. The cloud system water vapor content value near the outer edges of the typhoon is 6.01g·cm^-2, greater than other cloud. Average exchange times of water vapor content is 3.04 times per month in vertical atmosphere columniation and updating ratio of water vapor is 9.92 days. Average net water vapor content into the atmosphere in Zhexi drainage area in one year is 23487 million m³. It is possible that precipitation enhancement gross value would be about 2984.3 million m³,if all potential clouds in the sky in this area are seeded.