In order to explore the impact of the project of returning polders to lakes on the water environment, taking Luoma Lake as an example, based on the measured data, a two-dimensional unsteady water environment model was established for Luoma Lake before and after the implementation of returning polders to lakes. Combining the hydrodynamic model, Lagrangian particle tracking and water quality model, the impacts of the project of returning polders to lakes on hydrodynamics, water age and water quality were predicted. The results show that after the implementation of returning polders to lakes, under the conditions of easterly and northerly winds with a speed of 3 m/s, the average water level of the lake decreases by 0.36 m, and the average flow velocity increases by 12% at four monitoring sites. Under the condition of a northerly wind with a speed of 5 m/s, the water age near the polder area is reduced by 55%. The average improvement rates of COD, ammonia nitrogen, total nitrogen and total phosphorus can reach 3.3%, 3.1%, 2.3% and 2.2%, respectively. Therefore, it is concluded that the implementation of returning polders to lakes has a positive influence on the water environment of Luoma Lake.