Abstract: Under extreme weather conditions, such as storm surges or strong winds, high-intensity sand transport at the bed bottom is prone to occur, which has a profound impact on the dynamic geomorphological processes, port and waterway facilities, and ecological protection.It is of significant theoretical and practical engineering importance to conduct research on the mechanisms of sediment transport near the bottom.In this study, silt transport under wave actions was studied via wave flume sediment transport tests conducted in varying dynamic environments.A bottom sediment measurement system with a high sensor density was constructed to collect data from the fixed bed surface to the suspended sediment layer.The results showed that sand ripples on the bed surface developed under relatively weak dynamic conditions.With the increase in dynamic excitation, ripples were eroded and sheet flow occurred.The bottom layer movement of fine sediment, revealed by analyzing the sediment concentration process at different bed heights, was characterized by the silt surface lateral oscillations aligned with the periodic wave motion.The amplitude of the silt layer oscillations decreased with increasing depth and a phase lag arose.The concepts of movement number and compactness coefficient were introduced and a formula for critical sheet flow condition for a silt-sand wide gradation range was established.The bottom movement mechanism of fine-grained sediment proposed in this study will form a departing point for further studies of the mechanism of sudden-onset scouring and silting.