Based on the grey white function generation of Grey System Theory, a whitened differential equation has been established. It can be applied to forecast the discharge regime at the Three-Gorge Project (TOP) Dam site under flood and dry seasons. According to the variations of discharge with/without building TOP, the corresponding influence upon the marshland around Dongting Lake has been forecasted. In the Grey Syetem Theory a sample information curve is plotted first. After finding a series of time parameters at the curve on the basis of definite values, the GM (1.1) model group of the time parameters is set up. Different regimes such as 1974-type and 2000 year are considered. Based on the forecasted discharges before/after building TOP in Chenglinji Hydrological Station, the marshland decreases in East，West, South Dongting Lake areas are predicted respectively. Preliminary results show that there will be a 12. 1%-24. 9% decrease for the 1974-typo,a 17. 7. 1 %-26. 0% decrease for the 2000 year. Further analysis reveals that the decreases of marshland turns out to be greater in West Dongting Lake area than those in East Dongting Lake area.