Abstract: The variation rules of tidal flat at different scales has always been one of the main contents in the study of surface processes. The high-precision instruments ADV, CTD, and OBS were used to observe the scouring and silting characteristics of ebb and flow and spring-neap tide on May 26 to June 5, 2020 in Sansha Bay, Fujian, SE China. Results show that during the tidal cycle, the scouring and silting of the tidal flat are characterized by scouring at the beginning of rising tide, silting at the end of falling tide, and stabilizing in the middle period, the average scouring was 4.05 mm within 30 min when water depth is less than 1 m at the beginning of flood tide, and the average siltation was 3.72 mm within 30 min when water depth was less than 1 m at the end of ebb tide, and was in dynamic stability in the middle period. During the spring-neap tide period, the average siltation from mid tide to neap tide was 3.4 mm, and the average scouring from neap tide to spring tide was 8.2 mm. The correlation analysis on impact factors including velocity, turbulent energy, water depth, and suspended sediment concentration to the scouring and siltation showed that in tidal cycle, fluctuations in velocity and turbulent kinetic energy determined the changes of scour at the beginning of flood tide and siltation at the end of ebb tide. During spring-neap tides, the characteristics of more flood scouring than ebb siltation determined the scouring from spring tide to neap tide, and less flood scouring than ebb siltation determined the siltation from neap tide to spring tide. This study on scouring and silting mechanism in ebb and flow periods and spring and spring tides provided a reference for the study of tidal flat dynamic geomorphology and dynamic sedimentology.