Dongting Lake, as a reservior lake of Yangtze River, plays an important role in mitigating the downstream peak water levels of Yangtze River and reducing the threat of floods in the Yangtze River mainstream. This article analyzes the changing characteristics of Dongting Lake's storage capacity by constructing indicators that describe the flood-regulating characteristics of Dongting Lake. The research results show that before 2003, the peak-cutting rate of Dongting Lake remained at 38%, and the average peak-storing time was about 6 days. However, since 2003, the average peak-cutting rate of Dongting Lake has decreased to 35%, and the average peak-storing time has been shortened to 5 days. In addition, the annual maximum 15 and 30 day flood-regulating rates have also declined, but the peak-storing time remains basically stable.