Abstract: The asynchronous propagation of flood and sediment peaks has a significant impact on riverbed evolution. Since the impoundment of the Three Gorges Reservoir, sediment in the middle reaches of the Yangtze River has become more concentrated in individual flood events. Understanding the asynchronous movement of flood and sediment peaks during these events is critical for gaining deeper insights into the scouring and deposition adjustments of downstream river channels. However, the current understanding of the changes in the asynchronous propagation of flood and sediment peaks before and after the impoundment of the Three Gorges Reservoir, as well as their causes, remains limited. In this paper, the movement of flood and sediment peaks and the reasons behind their asynchronous nature before and after the operation of the Three Gorges Reservoir was analysed on the basis of observed hydrological data from 1993 to 2020 for the Yichang—Jianli reach of the Middle Yangtze River. The results show that before reservoir impoundment, sediment peaks in the Yichang—Jianli reach typically lagged behind flood peaks. After impoundment, this lag became more pronounced in Yichang station especially during the regulation of small and medium floods. As the sediment-replenishing effect downstream of the dam weakened the downstream sediment peak lag during the downstream propagation process. This study provides a scientific basis for the flood prevention safety in the middle and lower reaches of the Yangtze River and operation and management of the Three Gorges Reservoir.