Abstract: Surface sediments samples were collected from 106 stations in the outer Pinghai Bay and analyzed by laser particle size instrument for the composition and distribution of grain size parameters. The grain size trend model is used to discuss the transport trend of marine sediment， and the contrastive analysis of new and old maps based on ArcGis is used to analyze the erosion and deposition variation in Pinghai Bay. According to the Pinghai Bay Development Planning， representative stations are arranged to calculate the erosion of seabed sediments caused by tidal current and wave. The results show that surface sediments 5 types of which the clayey silt is the most important. The distribution of sediment types trend to finer with the increasing water depth. The average particle size of sediment （M_z）， sorting coefficient （σ_i）， skewness （S_ki） and kurtosis （K_g） are corresponding to marine sedimentary environment and hydrodynamic conditions. Sediment transport is mainly focused on the northwest and southeast direction. Chart comparison and the observations show that the area investigated is in a state of weaker siltation. The analysis and calculation proves that sea bottom sedimentation is not affected by tidal current but the waves．