In response to the lack of precise quantitative research on the impact of fishery-light complementary projects on the water environment, a hydrodynamic-water quality-water ecology-engineering coupling numerical model was constructed in Liuyanji Reservoir. The spatiotemporal distribution of mass concentration of water temperature (T), chemical oxygen demand (COD), dissolved oxygen (DO), total phosphorus (TP), total nitrogen (TN), and chlorophyll-a (Chl-a) in the reservoir area was simulated after the implementation of the fishery-light complementary project in August 2020, and the eutrophication index of the water body was calculated. The results showed that after the implementation of the fishery-light complementary project, T and the mass concentrations of DO and Chl-a in Liuyanji Reservoir decreased by 0～6 ℃, 0～0.02mg/L, and 0～0.0005mg/L, respectively, while the mass concentration of COD increased by 0～10mg/L. The mass concentrations of TP and TN varied very differently in the water area under photovoltaic panel than in the water area without panel. The mass concentration of TP in the water area under photovoltaic panel increased by 0～0.814 mg/L, while the mass concentration of TN decreased by 0～0.125 mg/L. The differences in water quality indicators before and after the implementation of the fishery-light complementary project gradually decreased with time, and the eutrophication degree of the water at the inlet significantly decreased. The eutrophication index of the water at the irrigation intake in the central reservoir area had increased by 5.61～16.00.