Lakes are highly susceptible to eutrophication due to their long-term closed or semi-closed status and poor hydrodynamic conditions. This paper constructed a hydrodynamic-convective diffusion model with MIKE21 software by considering the influence of rainfall on the spatial and temporal distribution of TN concentration in the Yilong Lake. The model simulated the hydrodynamic conditions under different inlet opening and closing conditions and different flow diversion schemes. The improvement of TN concentration based on the flow velocity distribution of the lake flow field was discussed. The results show that the hydrodynamic conditions and the water quality of the lake can be effectively improved by setting up water diversion inlets and increasing water diversion flow. The overall effect of diverting water from the Chenghe alone is better than diverting water from the Chenghe and Longgang River together. Under the scheme of separate diversion from the Chenghe, when the water diversion flow was double, the overall flow rate of the lake was increased by 44.04% and the TN concentration decreased by 52.78%, with the best effect of hydrodynamic optimization and water quality improvement, but the efficiency of water quality improvement per unit of water recharge is reduced. This research could provide some references for similar projects.