Abstract: Large scale hydrological observations in the Changjiang Estuary have been respectively carried out in February and July,2003.The survey range extends from Jiangyin to -20 m iso depth out of the mouth,and the measuring stations distribute in four outlets.Field observations show that:(1) temporal and spatial variation of salinity is mainly controlled run off discharge,flow distribution ratio of the outlets,tidal range and geomorphologic situation;(2) the order from high to low for the distribution of salinity is the North Branch,South Passage,North Passage,and the North Channel;(3) the saltwater flows backwards from the North Branch to the South Branch during the dry season,but more than half section of the North Branch is still held by freshwater during the flood season;the mouth sector of the North Channel,where tidal variation of salinity is major and seasonal variation is minor,has stratified interface between saltwater and freshwater;the South Passage,where tidal variation of salinity is minor and seasonal variation is major,is characteristic of mid strongly mixed type;and the situation of the North Passage is between situations of the North Channel and the South Passage;(4) the temporal and spatial variation of salinity reflects that flow distribution ratio to the North Branch,the North Channel and the South Passage increases to a certain extent in the flood season.