Abstract: According to the grain size data of 15 groups of surface sediment samples in wet and dry seasons, the methods of Pejrup grain size triangle, grain size-standard deviation curve and grain size parameter analysis are used to analyze the sedimentary characteristics of Ninghai-Xishuanhe lobe tidal flat in the south of modern Yellow River Delta in wet and dry seasons. The results indicate that the grain size of sediments change with season and the spatial distribution is different as well. Due to the large precipitation in wet season, the general sedimentary environment is stable, while the sedimentary environment in dry season is relatively turbulent, more obviously affected by the modern Yellow River. The correlation of grain size parameters shows that the sediments undergo long-time sorting before formation. The diversity of material sources has little to do with tidal flat dynamics, and the material sources are different in wet and dry seasons. By comparing the function values of sedimentary environment in wet and dry seasons, it is found that the wet season is strongly affected by tidal residual current, with neritic sediments dominated, while the dry season is dominated by fluvial (delta) deposits under the influence of Yellow River and nearby rivers.