[Objective] Environmental flow components (EFCs) are directly related to lake aquatic ecosystems and aquatic organisms, and can effectively evaluate the impact of flow variation characteristics on the environment and ecosystems. This study aims to clarify the characteristics of environmental flow variations in Dongting Lake after the operation of the Three Gorges Project, analyze the impact of these variations on the ecosystem of Dongting Lake, and provide a scientific basis for the management of the lake’s environment and ecosystem. [Methods] Based on the daily flow data at Chenglingji, Xiaohezui and Shiguishan stations in Dongting Lake from 1980 to 2020, Morlet wavelet analysis and the IHA software platform were used to analyze the runoff cycles and EFCs in the lake. [Results] Based on the wavelet analysis from 1980 to 2020, three cycles were identified at the Chenglingji, Xiaohezui, and Shiguishan stations, with a dominant cycle of 14 years for all. The runoff patterns between wet and dry periods showed consistent alternation in the outlet of Dongting Lake, the inlet from West Dongting Lake into South Dongting Lake, and the tail of the Lishui River. After the operation of the Three Gorges Project, large flood events still occurred in Dongting Lake. At Chenglingji, Xiaohezui, and Shiguishan stations, the peak flow values of these events increased to some extent, while their duration shortened. The frequency of small floods and high-flow events declined. Additionally, the minimum values of extreme low flows increased, and the timing of these values advanced. Combined with existing research findings, the homogenization of environmental flows in the Xiangjiang River and Ouchikou, and the diversification of those in the Zijiang, Yuanjiang, and Lishui Rivers, had no significant impact on the diversity of environmental flows in Dongting Lake. This was attributed to the regulation of Dongting Lake and the counterbalancing effect resulting from the convergence of different tributaries within the lake area. [Conclusions] It is recommended to increase the duration and frequency of high-flow releases during the spawning period of Chinese carp from April to July in Dongting Lake.