To quantitatively study the impact of the inflow of Dongting Lake and Poyang Lake on the erosion of the middle and lower reaches of the Yangtze River, based on the water and sediment data of the upstream main stream and the inflow of the two lakes, the average water flow erosion intensity over the past five years was used to characterize the water and sediment conditions. A formula for calculating the cumulative erosion of the reaches considering the inflow of lakes was established, and the contribution rate of the inflow of the two lakes to the cumulative erosion of the study reaches was calculated. The results show that after the application of the Three Gorges Project, the inflow of Dongting Lake and Poyang Lake accounts for 45% of the downstream runoff at Datong Station from 2002 to 2023, and the sediment content accounts for 22%. The cumulative erosion amounts of the Chenglingji-Hankou and Hukou-Datong reaches are 510 million m³ and 660 million m³, respectively. The calculated cumulative erosion amount of the research reaches fits well with the measured values, and the determination coefficients are both greater than 0.92. The contribution rate of Dongting Lake’s inflow to the cumulative erosion of the Chenglingji-Hankou Reach is 3.3% to 87.4%, while the contribution rate of Poyang Lake’s inflow to the cumulative erosion of the Hukou-Datong Reach is 6.6% to 19.7%.