MODIS images of 250 m resolution data were employed to know the spatio-temporal variation of water areas in East Dongting Lake and evaluate the impact of Three Gorges project on it. The water areas of East Dongting Lake and Three Gorges Reservoir were extracted every even month from a long-term period of 2002–2012. The spatio-temporal variation of water areas of East Dongting Lake was analyzed, and the distribution of seasonal hydro-fluctuation belt in East Dongting Lake was discussed. Based on the comparison of water areas variation between the East Dongting Lake and Three Gorges Reservoir, a possible way to assess the impact of the Three Gorges project was proposed. The results showed that the variation of water areas in East Dongting Lake was maximal in summer, and next in spring and autumn, while, it was minimal in winter. The hydro-fluctuation belt of East Dongting Lake mainly distributed in the southern regions of the lake during 2002 and 2012. The water areas in Three Gorges Reservoir and in East Dongting Lake showed a positive correlation in summer and in winter, but they showed a negative correlation in spring and in autumn.