Abstract: The rapid siltation of deep trough means the suspended sediment sinking very fast to the bottom of the trough within a quite limited period,and it makes up of fluid mud layer.A comprehensive analysis is done based on the data of duplicate measurement of seabed profile depth,topogr aphy,hydrology and suspended sediment in the Hangzhou,Sanmen and Xiangshan Bays.The results show that rapid accumulation happens under three conditions:(1) fine sediment is very abundant;(2) hydrology diminishes quickly;and(3) there exists a obviously concave landform.These three factors must be satisfied at the same time, other wise the rapid siltation cannotoccur at the deep trough.Since the Hangzhou Bay is full of fine sediment,the suspended sediment concentration is high all the year and it is influenced by the secondary Changjiang plume front and the Qiantang plume front at the east trial dredged trough,the rapid deposition can be measured when the hydrology turns weak from spring to neap tidal period.The suspended sediment is plenty comparatively in the Sanmen Bay,the seabed of tidal flat is kept stable with a little siltation and the trough is nearly in the equilibrium of erosion and siltation wit hina normal year.Only when the storm hits the bay and the following day sare between middle and neaptidal period after the storm past,a large number of suspended sedimenter odedobvio usly from tidal flat will sink quickly to the deep trough.However,the Xiangshan Bay is sheltered with hills around the bay and islands in bay mouth,the water is very limpid and the suspended sediment is quite little,so the rapid accumulation cannot be happened in the innerbay trough even with the impact of storm.