Lake Dongting is an important regulating lake in the middle reach of Yangtze River. It receives the volume and sediment of floods from Xiangjiang River, Zishui River, Yuanjiang River, Lishui River and Yangtze River, which leads to the flood elevation running up because of deposit. Certainly the flood disaster in the area becomes serious, the capability of water self-cleaning and water environment bearing become lowered, which leads to the environment worsened. This paper analyzes the effect on the flood elevation of Lake Dongting dredging according to the data from the Plan of Lake Dongting dredging and the typical reach dredging completed.