The hydrological observations in both spring and neap tides are carried out in February and August respectively. Based on these data , this paper studies the changing characteristics of water , sediment and salinity from the south channel to the north passage. Conclusions are drawn as follows: 1) The ebb tide duration is longer in the flood season while the rise tide duration is longer in the dry season on the whole. 2) The average flow velocity of the ebb tide is generally bigger in the flood season while the flood current speed is generally bigger in the dry season. During the period of neap tide, there exists the phenomenon that the average velocity of the flood tide near the bottom is bigger than the surface. 3) The value and vertical difference of sediment is larger in the flood season from midstream of the north channel to the water area outside. From the south channel to the upper reach of the north channel, the value of sediment is larger in the dry season. 4) There exists saltwater wedge in the midstream of the north channel which is upper in the dry season.