Abstract: According to the field data collected in the Changjiang Estuary in December 2011 and January 2012, there was a freshwater extension from the North Channel(NC) to the mouth and lower reaches of the North Branch(NB) where the water salinity was decreased during spring and the subsequent middle tides. However, the phenomenon was absent during the neap tides and subsequent middle tides. To reveal the origin of the freshwater in the lower reaches of the NB, We employed the well validated 3-D numerical model to reproduce the observed phenomenon. The numerical experiments were conducted and the freshwater flux decomposing method was used to investigate the underlying dynamic mechanism of the freshwater extension. We found that the freshwater in the NC extends to the NB mainly through its northern outlet(NONC) and neighboring shoals during spring and the subsequent middle tides due to the Lagrangian transports. Deepening of NONC facilitated the freshwater extension, and hindered the saltwater intrusion into the NB. In the contrast, the high-speed northerly winds would restrict the freshwater extension, while assist the saltwater intrusion into the NB.