Lake ecological water level (EWL) is an important factor to maintain the health of lake ecosystem. Based on the analysis of the daily average water level series of Chenglingji, Yangliutan and Nanzui hydrological stations in Lake Dongting from 1959 to 2016, the hydrological variation characters of Lake Dongting were determined by Mann-Kendall method, cumulative anomaly method and sliding T test method. Based on the method of annual EWL distribution and the improved method of IHA-RVA, the minimum and optimal EWLs were calculated, and the Tennant method was used for the reasonability verification. Finally, the guaranteed rates before and after the abrupt point were analyzed. The results show that: (1) The average annual water level of Chenglingji and Yangliutan hydrology stations were both on the rise, with much significant trend for the former one. While the average annual water level of Nanzui station exhibited significant decline trend. (2) The time of abrupt changes in water levels of the three stations was 2003, which was basically consistent with the beginning operation time of the Three Gorges Dam. (3) The annual minimum EWLs of Chenglingji, Nanzui and Yangliutan stations were 21.41, 28.95 and 27.84 m, respectively, accounting for 86.3%, 95.9% and 95.7% of the multi-year average level. The optimal EWLs were 23.29, 29.51 and 28.36 m, respectively, accounting for 93.9%, 97.8% and 97.5% of the multi-year average level. Such defined EWLs could meet the requirements of lake ecosystem goals with the consideration on the annual changes in natural lakes. (4) the safeguarding on the minimum ecological water level in Lake Dongting is relatively high, basically reaching more than 80%, but the guarantee degree of the appropriate EWLs is relatively low. After 2003, the guarantee degree of Lake Dongting decreased significantly in October and November, which is related to water storage and conservancy projects in the upstream. The EWL guarantee measures in Lake Dongting were suggested to maintain the health and biodiversity of the lake ecosystem based on the low guarantee degree and the difference of annual EWL distribution.