This paper, based on the historical literature and field observations, outlines the formation and evolution of Hongze Lake and the Huaihe River mouth along the lake.Hongze Lake was the result and component part of the engineering of regulating the Yellow River and protecting the Grand Canal.The present Hongze Lake mainly formed at the end of the 17th century, when Jin Fu harnessed the Yellow River and the Grand Canal by lifting the water level of the Huaihe river.After the lake formed, its level began lo rise continually at an average rate of about 4,0cm.a 1 for more than 100 years. At the same time, the lake level changed with an annual range of 4.lm, which resulted in strong backward flow in the upstream, whose uplimit moved backward gradually. Accompanying the lake-level rising and lake-volume increasing, the lake current changed evidently, especially the Hongze Lake influent-effluent current became weaker and weaker from the 18th century. Although there were absolute proofs about the sub-delta and mouth sand bar in the 18th century, the present Huaihe River delta along Hongze Lake was formed since the 1850s AD under the stable lake level and current. In 1128-1851 AD, the lower Yellow River captured the Huaihe River and a great quantity of sand was accumulated in the middle Huaihe drainage, some of which was carried into the Huaihe River channel afterwards, and became the Huaihe delta, s material base. The river counter-S-shaped valley from Fushan down to the lake has contributed to the river current wearing and sand accumulating. The evolutional model of the estuary may be summarized as follows:(1) formation of shoals;(2) fluvialization of the lake; and (3) anastomosing of the channel.