Based on the hydrographical characteristics of Dongting Lake, hydrological alterations are analyzed and ecological water-level are calculated by using the monthly water level data extracted from the 3 hydrological stations of Dongting Lake. The results indicate that: (1) The variations of diversion flow of 3 diversion outlets of Jingjiang river and the water level of the main stream of the Yangtze River caused by Jingjiang artificial cut-off project and Three Gorges projects are the main reason for hydrological alterations of eastern, southern and western Dongting Lake. And the variations of the water level for the main stream of the Yangtze River play a leading role in the hydrological alterations of eastern and southern Dongting Lake. The variations of diversion flow of 3 diversion outlets of Jingjiang river play a leading role in the hydrological alterations of western Dongting Lake. (2) The impounding effect of Three Gorges project, which leads to decrease of the diversion flow of 3 diversion outlets of Jingjiang river and the water level of the main stream of the Yangtze River, cause a significant reduction of the frequency for monthly water-levels meeting ecology water-levels in Dongting Lake; Jingjiang artificial cut-off project, which leads to decrease of the diversion flow of 3 diversion outlets of Jingjiang river and uplifting of the water level of the main stream of the Yangtze River, cause varying degrees of improvements for the frequency in eastern and southern Dongting Lake and reductions for the frequency in western Dongting Lake; (3) By analyzing hydrological alterations on monthly scale with t test and Mann-kendal test methods, and finding the probability distribution function for Water level sequence before variation with Kolmogorow-Smirnow method, monthly ecological water levels are calculated for eastern, southern and western Dongting Lake. The results is reasonable and could directly describe the monthly water level variation for three hydrological stations, and could provide some guides to the restoration of the Dongting Lake from time and space dimensions.