Water diversion and regulation is an emergency measure to improve the water quality of local Taihu Lake. The effectiveness of the measure is proved through experiences and studies for years. Because of the complexity of water diversion works and some impacts of uncertainties, the diversion project may bring some impacts on water environment along the diversion channel while it improving some local water environment. Especially in the rivers west of Wangyu River, some flows are blocked off. So the risks of the retention and transfer of pollutions in the Wu Xi Cheng Yu Rivers will rise up. According to the four drainage programs, by analysis of the hydrodynamics change and impact comparison of diversion project on water quality change in the west of Wangyu River, the retention time, intensity and transfer scope, and the risk of the polluted water were assessed and the corresponding emergency measures were proposed.