Lake Dongting, which is the second biggest fresh water lake in our country, is located in the middle reaches of the Yangtze River. For a long time, the Yangtze River and the other four riverways which inpour in Lake Dongting always take a great deal of sands every year, as a result, the lake was filled up, many continents were developed,the water velocity was slowed and the water environmental quality was worsened. In order to resolve these problems, dredging engineering is actualizing in Lake Dongting now. Combined with the engineering, the changes in C0D_Mn capacity, TN capacity and TP capacity of Lake Dongting before and after dredging were simulated based on the analysis of the Lake Dongting actuality water environmental quality in the paper, and the reasons were also analyzed. It was found that dredging engineering enhanced the water environmental capacity of Lake Dongting. Especially in the lower water and higher flux, the water environmental capacity increased more. Under the same source of explicit pollution, the dredging was helpful for improving the water quality of Lake Dongting, and it could ameliorate the eutrophic state of Lake Dongting in a certain extent.