Once the water level of the reservoir of the Three Gorges rises above 175 m, Yangtz River in Kaixian County will be changed into a lake-like reservoir which will be rare in the Three Gorges areas, and water pollution would be seriously deteriorated. In order to testify whether water quality meets the requirements of the function divisions, two-dimension water dynamics quality model and homogeneous-mixed water quality model are used to simulate polluted water disposal of Kaixian County before and after sewage purification. The analysis indicates that sewage disposal without any pretreatment will cause serious and continuous pollution; while sewage disposal after purificaiton up to standard, there are two kinds of results: in abundant and normal period , the water quality improves obviously, the water quality can meet the function of river area, but in dry seasons, the water quality improvement is not so effective, and still exceeds the control standard of the water function. To guarantee water quality of Xiaojiang River, it is proposed that, first, sewage treatment plant should run normally and continuously; second, other measures must be added to control sewage discharges in dry season.