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It is vital to employ efficient methods to calculate and determine the threshold of river ecological flow. These methods help to get the river ecological status control countermeasures in the plain river networks and construct a flawless long-term mechanism of ecological flow control (water level). According to the river's current environment and the actual circumstances surrounding its operation and administration, the upper, middle, and lower reaches make up the three sections of the Zong-Liutang River in the Plain of Northern Jiangsu. Furthermore, the ecological water level governing factor for each reach was analyzed and studied. The ecological water level threshold in the upper and lower reaches of the Zong-Liutang River adopted the specified values of Jiangsu Provincial Department of water resources. The threshold of the middle reach was thoroughly analyzed by using the determination method with multi-criteria, which consisted of the hydrological and hydraulic methods (including yearly minimum water level with a 90% assured rate method; wetted perimeter method; minimum monthly average water level method), aquatic living conditions method, and river landscape entertainment factor method. Taking into account the hydrological and hydrodynamic characteristics, observed hydrological data conditions of rivers, and the actual operation rules of rivers and water projects regulations in the Northern Jiangsu plain. Through comparing and contrasting actual operation and management plans, the target values of the ecological water level for the river's upper, middle, and lower reaches were 17.4 m, 8.96 m, and 7.5 m, respectively. Based on the river's ecological water level's control value, which was found through a thorough analysis, and the river's real operation and management conditions, specific measures to protect the river's ecological water level were suggested. These included developing a perfect and workable ecological flow control method, speeding up the development of an ecological water level goal assurance early warning system, and an ecological flow and inlet system.