Abstract: The peak flow is an important indicator of flood characteristics, and changes in its composition are crucial for reservoir scheduling. Research on the composition of incoming flood peaks of the Three Gorges Reservoir has focused mainly on periods of three days or more, with less attention given to the study of the peak inflow composition. In this paper, measured rainfall and flood data from the upper Yangtze River for the 1976 to 2020 period were utilized to statistically analyze the composition and variation in peak inflow to the Three Gorges Reservoir and to explain these variations in terms of rainfall. The results revealed that as the flood level increased, the contribution of the Jialing River increased, the contributions of the Min and Tuo Rivers remained stable, and the contribution of the Jinsha River decreased. When the peak flow was less than 30 000 m³/s, the Jinsha River contributed the most (34%); when the peak flow exceeded 40 000 m³/s, the Jialing River contributed the most (42%); and within the 30 000 and 40 000 m³/s range, the dominant tributary changed during different periods. Except for the Jinsha River, of which the inflow is affected by both rainfall and reservoir regulation, the variations in the flows from the other tributaries are positively correlated with rainfall. Significant variations in rainfall in the Jialing River basin during different periods lead to high fluctuations in flow, which is the main cause of peak inflow composition changes.