Poyang lake is the largest freshwater lake in China.During the low water level in autumn and winter, ecological environment of wetland in the delta front of the Ganjiang River consists of grass bank, muddy beach and back depression, providing favourable conditions for migratory birds to survive their winter. The investigation shows that the wetland at altiude of 12-13m is the most favourable place for migratory birds7 wintering. However the building of Three-gorges Reservoir may alleviate the discharge of the Changjing River in October, thus, may lead to the falling of the water level and to the exposedness of the lake bcach earlier than expected. The above-mentioned results exert less influence on herbivorous birds, but may reduce the numbers of animal foods slightly, the grass bank resources may be used not only earlier, but alse fully, because of the expanding of the lake beach, but the shelter condictions for migratory birds' wintering may get worse.