The proposed Yima Bridge acrosses the Zhushan Bay of Lake Taihu,the water blocking effect of the bridge piers can reduce the exchanged water flow and affected water quality. This paper adopted a two-dimensional unsteady numerical model of nested grids to simulate the change of hydrodynamic and water quality after bridge construction and to quantify it. The simulation results show that under the effect of continuing 5.0 m/s southeast wind: ① the hydrodynamic conditions of Zhushan Bay is weakened, and the exchanged water flow decrease about 30%~40%; ②The water quality deteriorates especially in northwestern shore, and the concentrations of COD,TP,TN increase average about 0.9, 0.001, 0.1mg/L.