Abstract: Based on the observed hydrological data and sediment in winter and summer in the Funing Bay and its nearby sea areas in the northern Fujian, South China, we analyzed the characteristics of suspended sediment grain size at different time scales, discussed the main factors affecting the seasonal variation of grain size, and revealed the relationship between grainsize and concentration. Results show that in normal weather, the median grain size is generally larger than 6.5 Φ in the study area. The grain size shows seasonal distribution characteristics: being fine in winter and coarse in summer. Under the influence of tidal power, the grain size changes significantly between spring and neap tides. The variation of grain size in tidal cycle is not obvious. The standard deviation of grain size curve shows that the distribution ranges of two main sensitive grain size are 2.8～3.4 μm and 21.1～25.1 μm. The seasonal variation in grain size is mainly affected by sediment resuspension and source supply. Correlation between median grain size and concentration in winter is not obvious, but there is a significant linear relationship between them in summer. With the increase of concentration, the grainsize becomes finer. This seasonal variation may be related to the sources and mixing of suspended sediment. The coarsening of grain size in summer may be related to the flocculation settling of fine particles.