Water-level fluctuation (WLF) is regarded as a key environmental factor for lake ecosystems. Keeping moderate WLFs approximating to its natural conditions is substantially essential to maintain its biodiversity and integrity. In this study, a hydrological index based method for determining ecologically acceptable water-level range (EAWLR) was illustrated by a case-study of Dongting Lake. This method was proposed based on a consideration of hydrological alterations induced by natural variability and human activity. It was during this process that two hydrological indices, the ratio flow deviation (RFD) and the amended annual proportional flow deviation (AAPFD), played vital roles in change point detection and the determination of EAWLR, respectively. The WLFs are closely related to species richness. The relationship between them follows a hump-backed curve and EAWLR serves as the hump part of the curve. The final results indicated that EAWLRs of Beijinggang, Nanzui and Chenglingji during flood season were 27.78-38.26 m, 25.19-38.45 m and 24.48-30.96 m, respectively, while those during non-flood season were 18.33-27.79m, 17.68-29.19 m and 15.59-22.07 m, respectively. The response to the great flood in 1998 and the drought in 2006 of Dongting Lake region verified the rationality of the results. Within the EAWLR, ecosystems can operate normally and have stable community structures to resist external interference.