In order to highlight the importance of the ecosystem purification and human regulation in the carrying capacity of water environment, the concept and evaluation model of water environment carrying capacity was built based on the ecosystem pollution purification and human-regulated pollution reduction. As a study case, water environmental carrying capacity of Dianchi Basin was calculated in 2015. The results illustrated that the environmental capacity comprehensive index of Dianchi Basin was 1.16, which was in an overload state. Regarding the distribution of environmental capacity, the north was higher than the south. The limiting factor of the north and south were Chemical Oxygen Demand and Total Phosphorus, respectively. The transfer of pollutants in the water transfer project was equivalent to 16% of the water environment carrying capacity, which was very important for the improvement of watershed environment. The reduction capacity of human regulation has taken up more than 50%, which has become a non-negligible part.