To quantitatively assess the distribution of excess flood volume in Dongting Lake during the inflow of floodwaters from the Jingjiang River's three diversion outlets, as well as the Xiangjiang River, Yuanjiang River, Zijiang River, and Lishui River, we establish a regional flood routing model. Our model focuses on key control sections, namely the Luoshan station in the Yangtze River's main stream, the Qilishan station in East Dongting Lake, the Lujiao station in South Dongting Lake, and the Nanzui and Xiaohezui stations in West Dongting Lake. By calibrating the parameters using typical flood data from 1996, 1998, and 2017, we calculate the distribution of excess flood volume within Dongting Lake during the 1954 flood (defense target flood for Yangtze River) before and after the Three Gorges operation. The results highlight the apparent flood regulation effect of the Three Gorges reservoir. However, even when facing the target flood, an excess flood volume of 21.5 billion m³ remains near Chenglingji, with Dongting Lake bearing an excess of 14.3 billion m³, among which the West Dongting Lake, South Dongting Lake, and East Dongting Lake bear excess flood volumes of 2.6 billion m³, 6.2 billion m³, and 5.5 billion m³, respectively. This result is equivalent to the total flood storage volume of the storage embankments in the Dongting Lake area. The findings serve as a technical reference for optimizing flood control layout in the middle reach of the Yangtze River.