Through high-resolution research of sedimental chronology and the sediment environmentalindexes, such as graininess, minerals, magnetic parameters, pigment content, organic carbon andchronology in Ds-core and Ws-core in Nansihu Lake, the authors analyze the formation cause of theNansihu Lake and its water environmental changes. Historical documents are also analyzed here inorder to reach the conclusion. Researches indicate that the Nansihu Lake came into being about 2500aBP and its evolution succession can be divided into four stages. In this evolution process, severalscattered lakes merge into one large lake in the east of China. This process is distinctively affected bythe overflow of the Yellow River, the excavation of the Grand Canal and other human activities.