This thesis studies the saltwater intrusion in the Changjiang Estuary quantatively by using a 3D numerical model and the method of flux analysis. The impact of the saltwater spilling over from the North Branch, the deep waterway project, and the winter monsoon are considered. The main fruits are as following.1. A 3D, high resolution, moving intertidal zone, hydrodynamic and salinity numerical model in the Changjiang Estuary is built up. Several field-measured elevation, current and salinity data are used to validate this model, and the result is reasonable.2. The transport mechanism of the saline water that spilled over from the North Branch in the Changjiang Estuary is analyzed quantitatively by use of the flux analysis method. The results suggest that Lagrange residual current and tidal pump are the main dynamic mechanism for the saline water mass from the North Branch spills over into the South Branch and drifts downstream. In most places the former is dominated but somewhere else these two factors are in equilibrium. To investigate the transport route of the spilled saline water mass, the cross-section salinity flux over each waterway in the South Branch is calculated, leading to a result that the most proportion of it is transported downstream through the North Baimao Sand Waterway and the rest through the south one. The two parts join together near Qiyakou, then the fraction of it enters the Xinqiao Waterway through the Nanmen Waterway and is transported upstream by the flood current, while the left keeps on drifting downstream along the main channel in the South Branch. At the lower reaches to the New Liuhe Sand, the spilled water mass enters into the South and the North Channels through several waterways around the bifurcation, respectively. In spring tide, the magnitude of salt flux during the South Channel is more than that in the North Channel, while on the contrary during the neap tide.3. The net transetional water mass flux (NTWF) at the upper reaches of the North Branch is used to present the intensity of salt water spilled over, then its quantative relationship with runoff and tidal range is studied on the diurnal and semimonthly basis.In the diurnal case, the NTWF can be determined by 533.1×exp(0.361×Isso)-533.1 (Isso≥0) and 226.0×Isso (Isso<0), where Isso (index of SSO) is defined as TR-1.58×exp(0.2457×RF/10000), TR is the tidal range (m) at the Qinglong Harbor and RF is the runoff (m~3s~(-1)). The Isso has the same sign as the diurnal NTWF, when Isso is positive the NTWF points to the South Branch and the salt water spilling over takes place, while on the contray when Isso is negative. If Isso is equal to zero there is no net water mass transport existd.In the semimonthly case, the NTWF is determined by 701.89×ln(-RF/10000. +6.2)-553.54, which decreases with the increasing of runoff and declines to zero when the runoff reaches 40000 m~3s~(-1).The tidal range and runoff are measured at Qinglong Harbor Station and Datong Station everyday, respectively. Thus the NTWF can be easily obtained by using these