Based on topography, hydrology, tidal current, bed sediment and core data collected at the Lvsi sea area, an analysis of the bed-material grain-size variation processes and its effect of the waterway siltation is carried out. The results show that the bed-material armoring is shown at the Lvsi trial dredged channel after the waterway dredging. The median diameter of the bed-material is 0.045~0.071 mm before dredging, which increases to 0.128~0.135 mm after dredging. The sediment component between 0.005 mm and 0.062 mm decreases from 36.0% to 8.9%, and that between 0.075 and 0.250 mm increases from 37.9% to 78.2% by comparison with before and after dredging. The main course of the sediment composition change is the hydraulic sorting by the dredging. The silting forms combining with the suspended sediment deposition and bed load transport is a mechanism of the Lvsi waterway siltation. The siltation intensity of the trial dredged channel is great during the period from July to August in summer, and it is small during the period from November to December in winter. Furthermore, the medium diameter of the bed-material has become finer in summer and coarser in winter. The results of grain size spectral calculation show that the contribution rate of the suspended sediment deposition on the waterway siltation is about 15% in the normal condition, and the contribution rate in summer is higher than that in winter. However, this rate can be increased to 30%~40% in the windy condition. The calculation method of the contribution rate provides a reference for the siltation studies of the similar works in the future.