Abstract: The grain size distribution characteristics, the surface sediment transport trend, the sedimentary environment partition and dynamic mechanism in Zhao'an Bay were investigated with hydrodynamic observation at 6 sites, surface sediment grain size data of 181 samples and topographical data, using Gao-Collins particle size transport model and system clustering analysis. The results show that 6 types of sediments, mainly included clayey silt and sand, were found in Zhao'an Bay, where their particle sizes range from 0.54 φ to 7.43 φ with average 5.85 φ. The grain size is coarser in shallow water of the bay head and finer in deep waters at the bay mouth and the coarsest particles were found in the tidal channel at the bay mouth. Affected by multiple factors such as waves, tidal currents, runoff, topography etc., sediments at the bay mouth generally transport from the outer sea into the bay in direction to the northeast. Sediments in the southern middle part of the bay transport obviously from the shore to the waterways in the bay center. The sediment transport trend in the northern middle part is not clear. The sediments in the bay head show an obvious transportation toward the south. Considering the hydrodynamic conditions, geomorphological characteristics and sediment sources, 4 types of sedimentary environments can be distinguished, namely the bay mouth tidal channel landform, the bay middle tidal flat landform, the bay mouth nearshore shoal landform and the bay head tidal channel landform.