Based on the topography,bathymetry data and remote sensing image in different periods,we adopt the GIS technology to analyze changes of Daishan northwest coastline and the seabed erosion and silting offshore.Then local hydrological and sediment features and the human sea reclamation activities are combined to study the main influential factors on the sea erosion and silting of the northwest area of Daishan island in recent 30 years.Research shows that,before 2011,northwest Daishan coastline was stable,and the seabed had experienced a dynamic evolution process from silting on beach and erosion in trough to erosion both on beach and in trough.The erosion and silting-up rate were generally less than 10 cm/a in the sea area between Daishan island and Dayushan island,with a trend of overall increasing erosion rate,except for some local troughs where the seabed change was more intense.After 2011,due to the large-scale reclamation project,seabed shallower than 5 meters off northwest Daishan silted with a rate of about 1.75 m/a,while other sea areas were still rushed by erosion.The sediment route in Daishan northwest mainly depends on the transit transport.The reduction of sediment from the Yangtze River and the increase of the reclamation activity in recent years are the two main influential factors of the seabed sediment evolution of the waters.