Dadong Port, 40 kilometers southeast from Dandong city proper of Liaoning province, is the northeastern sea port of China. Up to now, we have been observing and accumulating much information about hydrology and sediment. Formerly, the hydrological survey and calculation was very hard to conduct. By means of analyses and comparision, the paper puts forward the formula to calculate the sectional sediment discharge rate with the tide flowing and ebbing rates as dynamic parameters,and to forecast the scouring and filling condition of the bay with the net sediment discharge rate. The formula will reduce the anount of work to collect and calculate original data, and to forecast the natural life of the bay through the calculation of sediment accumulation rate in a better way.