Abstract: Based on the stratified observation data of the Pearl River Estuary for half a month in the dry season from January 3 to 18, 2016, analysis of the mixing characteristics and mechanism of salt-fresh water in Lingdingyang and Modaomen estuary. The results show that: (1) Salinity stratification is the most obvious at the mouth of Lingdingyang and Modaomen estuary, during the neap tide salinity stratification is the most significant, the mixing strength increases gradually with the increase of the tidal range; (2) Salinity stratification of Modaomen estuary is more significant than Lingdingyang estuary, the reason is the difference of the upstream flow; (3) During the spring tide turns to neap tide, salinity stratification becomes obvious gradually in Lingdingyang estuary, while the salt fresh water in Modaomen estuary is mixed sufficiently, this is because although the salinity fell towards to the mouth but it fell slowly lead to some still stay in the river in Lingdingyang estuary, but in Modaomen estuary the salinity shakes violently and quickly exits the estuary; (4) In the mouth of Lingdingyang and Modaomen estuary, the inhibitory action produced by gravity circulation and tidal shear is greater than the auxo-action of tidal turbulence, so salinity stratification is significant; the water depth in the upstream of the estuary is shallow, so the tidal turbulence is strong that promoting the mixing of salt-fresh water.