The operation of the Three Gorges Dam has changed the flow and sediment conditions in the lower reaches of the reservoir, and it affected the ecological balance of the Lake Dongting and resulted in corresponding ecological problems. T-test was adopted for testing the consistency of the annual average water level based on the daily water level data of 1953-2018 at Chenglingji Station which is taken as the water level of the East Lake Dongting. The results show that the year of 2004 is an abrupt change point, indicating that the operation of Three Gorges Dam has resulted in the dramatic variation of water level at Chenglingji Station. Drawing on the IHA (Indicators of Hydrological Alteration) and RVA (Range of Variability Approach), an index system for calculating suitable ecological water level was presented in consideration of the range of monthly mean water level, the fluctuation range of annual water level, frequency and duration of high and low water level, and rate and frequency of water level condition changes. This index system can represent the requirements of the original ecological system intuitively and comprehensively. Using this index system, the suitable ecological water level of East Lake Dongting before the operation of the Three Gorges Dam was calculated for the following variables:1) the range of monthly mean water level (from January to December):17.07-18.34 m, 17.15-18.89 m, 17.65-22.23 m, 20.25-22.15 m, 22.85-24.90 m, 24.31-26.44 m, 26.88-29.16 m, 25.79-28.32 m, 25.12-27.56 m, 23.59-25.88 m, 20.65-22.81 m, 18.58-19.88 m, respectively; 2) annual acminimum water level:16.21-17.86 m, occurring from day 16 to day 51 (annual accumulative day); 3) annual maximum water level:28.54-31.48 m, occurring from day 187 to day 211 (annual accumulated day); 4) the high water level lasts 32.62-81.32 days at one time, the low water level lasts 52.13-107.65 days at one time, water level rises 21.9-26.45 times, the rate of water rising is 0.17-0.21 m/d, water level falls 23.17-27.6 times, the rate of water falling is 0.12-0.14 m/d. The results indicate that the monthly mean water level in January and February is 0.83 m and 0.27 m lower than the demand for suitable ecological water level, the monthly average water level in October is 0.83 m higher than the demand, the annual minimum water level is 0.39 m higher than the demand, occurring 6 days earlier than demand. The research results can provide a basis for the joint operation of Three Gorges and upstream cascade reservoirs.