The problem of sediment deposition directly affects the lifespan and comprehensive benefits of the Three Gorges Reservoir (TGR). It is of great importance to explore the characteristics of incoming runoff and sediment load in the TGR, which is conducive to solving the problem of sediment deposition in the reservoir. Based on the measured hydrological data, this paper analyzes the flow and sediment transport characteristics and source composition in the TGR, with special emphasis on such characteristics during the high flood period after the operation of the cascade reservoirs in the Lower Jinsha River. The results show that: The incoming sediment load in the TGR was mainly concentrated in the high flood period from 2003 to 2021, and this phenomenon was more significant after 2013. The main source of the sediment load has also changed from the Jinsha River to the Jialing River after 2013. Besides, for the high flood period in which the peak discharge at Cuntan Station was higher than 50000 m³/s, the sediment load in the TGR was significantly greater than those in the peak discharge intervals of 30000-50000 m³/s. The key periods for the sediment regulation of the TGR was during the numbered flood periods in the Upper Yangtze River. Additionally, the basin floods occurred in the tributaries upstream of the TGR, such as the Tuo River or the Jialing River, may cause the phenomenon of “little water carries massive sediment” happened in the TGR during the high flood period. After the operation of the cascade reservoirs in the Lower Jinsha River, the sediment load in the TGR during the high flood period has been significantly decreased, which has greatly reduced the amount of sediment deposition during the middle and small flood operation. This study can provide technical support for the refined sediment regulation of the TGR, and provide a solid guarantee for the long-term use of the TGR.