The environmental flow requirement is very important for the health of river and lake ecosystems. The sum of the environmental flow of river and the lake is often considered as the eco-environmental water demand of a river and lake coupled ecosystem in most studies. However, there may be problematic because the connection between the river and lake is ignored. Based on the water volume linkage between river and lake ecosystems, this paper constructed a watershed optimization model for a river and lake coupled ecosystem, and analyzed the eco-environmental water demand rules of watershed by taking Lake Dianchi as an example. The results showed that the environmental flow requirement of a single river or a lake could not meet the eco-environmental water demand of the lake basin, and the connection between rivers and lakes needed to be comprehensively considered. Under the current water quality conditions, the annual water transfer volume from Niulanjiang River cannot meet the eco-environmental water demand of Lake Dianchi Basin. The water pollution of Lake Dianchi Basin has a great impact on the eco-environmental water demand of river and lake coupled ecosystem in the basin. With the decrease of water pollution degree, the eco-environmental water demand for the whole system and water diversion in the basin decreases exponentially, and water use for river ecosystem decreases linearly.