The sediment of Lake Chenghu, located in the southeastern area of Suzhou, is mainly composed of ‘hard clay’ (a kind ofsediment consisting of silt and clay), and has been featured with lake depressions and a paleo-channel in the central area. Thepaleo-channel, along the direction of NNW-SSE, is accord with the Dongjiang River recorded in historical documents, and itssediments may become good archives of environmental evolution of the hard clay plain from river to lake. The grain-sizecharacteristics of core SC7 of Lake Chenghu, AMS 14 C and their magnetic susceptibility were analyzed in this paper, then thesedimentary characteristics and paleoenvironmental significance were discussed. The results indicate that the sediments of LakeChenghu mainly consisted of fine-medium silt and clay, and the percentage of fine-medium silt was about 53%-65.8%, clay about19.7%-40.1%. The grain-size parameters of the sediments above 336cm was different from hard clay, and magnetic susceptibilitywas significantly higher. Grain-size parameters and magnetic susceptibility of the sediments show that the sedimentary environmentof Lake Chenghu had experienced four main phases from river to lake after the formation of hard-clay plain, and also provided newevidence for the causes of river-flooding lakes in Taihu Plain. It also suggested that the formation of surface sediments had beenseriously affected by human activities.