In order to ensure the water quality of Longwangmiao Section of Chengnan River in Pukou District of Nanjing City to reach the standard, considering the hydrology, water quality, and pollution source status, the pollution source of the Chengnan River into the study area was generalized and the water environment mathematical model was established. The water environmental capacity of Chengnan River was calculated based on the water quality of Longwangmiao Section, and the reduction ratio of pollutants was determined by comparing with the pollutant quantity entering the river. The results show that the environmental capacity of COD, NH₃-N, and TP is 2 183 t/a, 201 t/a, and 154 t/a, respectively, under the condition that the water quality of Longwangmiao Section reaches the standard. And the difference between pollutant reduction rate and water quality exceeding standard rate is less than 20%.