Abstract: Effects of the runoff and the tidal flow on the invasion of salt water with four kinds of hydrologic conditions are simulated by use of a two-dimensional salinity mathematical model for the Yangtze River Estuary. Transportation laws of salinity with flow in the Lanmensha area of the Yangtze River Estuary are discussed. Influences of the inclined pressure item caused by salinity-gradient on the flow and those of the deepening of the channel on the invasion of salt water are analyzed by use of a three-dimensional salinity mathematical model for the north channel of the Yangtze River Estuary.