Hydrodynamic and water quality model for tidal river network is set up with MIKE11 modeling system, according to the features of tidal river networks in plain area. The model was calibrated using the hydrological and water quality data of 1999, and the results show that the simulated values agree with the measured data very well. This model is used to numerically analyze the effects of low flow augmentation on hydrodynamic and water quality conditions of Suzhou Creek. The simulation results show that the flow augmentation can increase net discharge of Suzhou Creek and improve its ability of re-aeration; and its concentration of dissolved oxygen in the river networks can also increase correspondingly.