In order to reduce the area of water level fluctuation zone in Yulin River caused by the dispatch of Three Gorges Reservoir, an eco adjustment dam has been constructed. However, this construction also changed the hydrodynamic characteristics of Yulin River, thereby changing the quality of the water environment. Quantifying the impact of this construction is worth studying. Based on SWMM（storm water management model） and EFDC(environmental fluid dynamics code) models, a coupling model system of hydrology, hydrodynamics and water quality of Yulin River Basin was built to investigate the influence of the eco adjustment dam on the hydrodynamics and water quality of Yulin River . The simulation results with and without the dam show that after the operation of the dam, the flow velocity of Yulin River greatly decreased, and the drop in the downstream section (from Shujia to the dam) was greater than that in the upstream section (from Meixi to Shujia). Obvious water quality segmentation occurred during the dry season because of the velocity decrease, but the phenomenon was attenuated in wet season. All indexes of water quality increased after the dam was built with the highest concentration occurring in the section flowing through the construction land and the largest change appeared in the head section of the reservoir. The simulation results can demonstrate the impact of the dam on the hydrodynamics and water quality of Yulin River and this research can provide some reference for assessing the effect of similar construction projects on other tributaries of the Three Gorges Reservoir.