In order to explore the ecological water requirement of rivers for different purposes, the case of Liuli River in Beijing Fangshan district was studied. The meaning and methods of calculation of environmental water requirement and ecological water requirement were defined. In this paper, environmental water requirement and ecological water requirement were calculated in the same project for the first time. Because the sewage treatment plant will be operated in 2021, sewage will continue to be discharged into Liuli River in 2019 and 2020. Before the operation of sewage treatment plant, the function of environmental water requirement is to eliminate sewage. However, after operation, the function of ecological water requirement would beto maintain the ecological balance. Both types of water requirement were calculated in this paper. The results indicated that both of the ecological and environmental water requirements were 1967800 m³. For eliminating sewage, the amount of purified water requirement was 303900 m³ considering water dilution and self-purification. The amount of landscape water demand was 1804300 m³. The amount of soil water storage was 161360 m³. The amount of annual evaporation from the river was 780300 m³. The amount of annual infiltration of the river was 216900 m³. For maintaining ecological balance, the water requirement for the habitat of aquatic organisms was 717300 m³. Future studies should be focused on the methods of coordinating the needs of different targets, refining the calculation methods of ecological water requirement, and maintaining a stable and healthy ecosystem under different situations.