In recent years, the water quality of Yangcheng Lake is always worse than class Ⅳ, the water environment is pessmistic and not suited to its strategic position. Qipu river which converge the lake with the Yangtze River is a very important river in Yangcheng lake aera. Combined with Qipu comprehensive renovation project, this paper designed the different conditions and studied the reasonable water diversion scale and water environment effects in application of numerical simulation methods. The result shows that the concentration filed approximately trends to be stable after 72 hours with the different diversion scales.When the flow increased to 100 m3/s, the improvement degree of water quality is most obvious.The time length of 72 hours and the scale of 100 m3/s diversion is the optimum diversion program. When the urgent pollution events happened after maintaining water diversion, the average velocity of current increased to 0.7 cm/s. The concentrations of TN and TP decreased 59.83% and 65.29% respectively. The improvement effect of water quality in Yangcheng Lake is obvious.