Analysis on the hydrologic characteristics of the main stream of the Yangtze River and Poyang Lake were studied to discuss the water exchange between the main stream of the Yangtze River and Poyang Lake before and after the operation of Three Gorges Reservoir, as well as in the typical dry year of 2006. The annual runoff distribution for dry season of Hukou station, located at the outlet of Poyang in 2000s has increased, compared to previous years. And the percentage of runoff in September and October has also increased, compared to 1990s in last century. The water exchange coefficient was 0.51 in the year 2006, which means nearly stable effect between the Yangtze River and the Poyang Lake. Meanwhile, the Poyang Lake provide water supply up to 1564 × 108m3, accounts for about 23% of the Datong runoff in the same period, and 5% more than the normal year. This is the main reason that the discharge maintained more than 10000m3/s at Datong all over the year 2006, reaching a positive phenomenon of ‘no drought in dry season’.