Abstract: In this paper， the characteristics of tidal dynamics in Sanmen Bay have been analyzed on water current observations in 2013. The result shows that Sanmen Bay is a strong tidal bay where the tidal is higher in top of the bay than the bay mouth. The tidal property belongs to regular semidiurnal tide and the amplitude of shallow water constituents comes to stronger toward the top of the bay. The flood/ebb time duration is similar and the flood/ebb current velocities are similar， too. The dominant current is between 45%～58%. Flow vector is parallel to the coastal line in the rectilinear current movement， and the bottom friction has significant effects on the vertical structure of tidal current. The flow rate in vertical direction is different remarkably and distributes logarithmically. It is the maximum in the surface or subsurface water and decreases toward the bottom with minimized value in the underlying water. At all the stations， the underlying flow rate is relatively concentrated. The more upper the layer， the more differences in the flow rate at all the stations. The largest velocity gradient appears in the time of rapid flow and the smallest velocity gradient appears in the time of slack water. Roughness of the sea bed at the bottom has great impact on the velocity gradient．