The seabed process of the Jiaojiangkou bay from 1931 to 2004 and its mechanism are studied by analyzing the hydrology,sediment and topography data.The results show that from 1931 to 1970,the shorelines of the Jiaojiangkou bay were advanced rapidly towards the sea,and siltation occurred widely in the Jiaojiangkou bay；After 1970,erosion was presented in the channel of the bay top,while siltation in the rest of the Jiaojiangkou bay became slight and the rest tended stabile,which mainly resulted from the reduction of incoming sediment,decrease of the shoals on both banks and effect of seawall’s converging.