A hydrodynamic-water quality model was applied to simulate the process of hydrodynamic and water quality of Poyang Lake before and after the construction of the scheduled Poyang Lake Hydraulic Project (PLHP) in extreme receding year (2016) and general receding year (2017). And then a water environment capacity model was constructed to assess the impact of the scheduled PLHP on the water quality and water environment capacity in the lake district. Simulation results show that the construction and operation of PLHP could significantly increase the water body volume of Poyang Lake with the maximum increase being 77% per month. It mainly reduced the TP mass concentration in the middle lake during the early period of water level recession. In the late period of water level recession, the TP mass concentration in the upstream of the gate increased in extreme receding year while the TP mass concentration in the upstream of the gate decreased in general receding year. However, there was no significant effect on the spatial distribution of mass concentration of CODMn and NH₃-N. The construction and operation of PLHP could increase the mass concentration of CODMn, NH₃-N and TP of the lake in extreme receding year, while reduce the TP mass concentration of the lake in general receding year. The construction and operation of PLHP could significantly improve the water environment capacity of CODMn and NH₃-N of the lake, while the water environment capacity of TP increased in general receding year and might decrease in extreme receding year.