Based on the theory of ecological irrigation district, this paper discussed the contents of the northern large-scale irrigation districts' water requirement for eco-environment, and defined it for the maintenance of healthy water requirement for irrigation districts' ecosystem. Using the system dynamics approach, it constructed a system dynamics(SD) model of water requirement for eco-environment of irrigation districts. Taking Baojixia Irrigation District in Shaanxi Province as an example, it also simulated the water demand for its eco-environment from 2005 to 2030. The results show that the water demand for ecological environment in Baojixia irrigation district increases at the rate of 6.08‰ per year. And this irrigated area is developing in the same direction as an eco-irrigation district.