Sediment movement patterns control the stability of sandy seabed in three aspects: (ⅰ)the sediment edibility time and the thickness of the moving layer; (ⅱ)seabed accretion/erosion and their rate; and (ⅲ)seabed elevation changes in response to bedform (e.g. underwater dunes and tidal ridges) migration. These problems can be solved by the following procedures. Firstly, the sediment erodibility time and the rate of bed-load transport are calculated using an appropriate equation which takes into account the combined effect of waves and tidal currents. Secondly, the moving layer thickness and the accretion/erosion rate are estimated using the transport rate data. Finally, the information on the migration velocity of dunes/ridges and the resultant seabed elevation changes is obtained using the transport data and the geomorphic data of the bedform. As an example for its usage, we applied the method to the assessment of seabed stability over an area to the west of Hainan Island. The results show that the seabed material is highly mobile, with active sediment movement. The rate and amplitude of seabed elevation changes are both relatively large, in response to rapid bedform migration. These characteristics must be taken into account in the development of the natural resources of the region.