According to the quantitative analysis on historical records and deposit evidences,Poyang Lake, formed in about 400 A.D., can be regarded as a relatively young lake.The direct and main reasons of its formation were due to the shift of the Yangtze River's main cause to the vsouth.As the river obstructed the out-flow of the Ganjiang River, its lake water was extended southwards to form Poyang Lake.The variation of the surface area and water level of Poyang Lake mainly depends on the fluctuation of the Yangtze River at the Hukou section.The evolution of Poyang Lake may undergo the following 4 stages: 1.During the Han Dynasty (206 B.C.-220 A.D.) Water in the northern lake basin used to be a relict part of the ancient lake (the Pengli), and no large water body existed on the ancient Ganjiang River Plain to the south of Yingzikou, where both Xiaoyong City and Haihun City were ever built.2.From the Six Dynasties (222-618 A.D.) to the early stage of the Tang Dynasty (618-718) Water body in the northern lake basin rapidly extended southwards to cover an area of about 6000km² (the highest water level).3.From the last stage of the Tang Dynasty to the Song Dynasty (618-1280A.D.) In the southern basin the lake delta developed rapidly and the lake began to descend.4.From the Ming Dynasty to the Qing Dynasty (1368-1911 A.D) The lake area enlarged again and the Ganjiang River mouth was submerged, as a result many tributaries were formed.