In response to the problem that the increase of ecological water supply in Xuanwu Lake from 2012 to 2017 cannot improve the water quality of the lake, the water quality optimization method and application of Xuanwu Lake were studied by fitting the measured water quantity and quality data, deriving the zero dimensional model and two-dimensional water environment mathematical simulation prediction. In the process of fitting of measured data, it was found that the water quality of the lake area first got better and then got worse with the increase of water supplement. Through the formula derivation of the zero-dimensional model and the method of quantifying the parameters, it was concluded that the degradation amount of pollutants in Xuanwu Lake had a negative linear function relationship with the water diversion. The lake-reservoir hybrid model was used to further deduce that the water quality of Xuanwu Lake showed a quadratic relationship with the water diversion amount, and it was concluded that the optimal water replenishment amount was 2. 0×10⁵ t/d during the wet season and 1. 5×10⁵ t/d during the dry season. The mathematical model of the water environment is used to simulate and predict, and the optimal replenishment amount is 1. 2×10⁵ t/d when the current wastewater into the lake is reduced by 40% during the dry season. During the wet season, the optimal replenishment amount is 1. 8×10⁵ t/d when reducing 40% of the current sewage into the lake in the near future. During the wet season, the optimal water diversion amount is 1. 5×10⁵ t/d when the current sewage into the lake is reduced by 80% in the long-term.