Based on the simultaneous observation data of tidal current and sediment content at 12 stations in the Sanmen Bay during dry and flood seasons in 2013，the mechanism decomposition method was used to analyze the transport mechanism of suspended sediment in the dry and flood seasons，and to explore the contribution rate of different hydrodynamic factors. Results show that：1）Whether it is the flood season or the dry season，the Euler residual current and Stokes residual current increase gradually from neap tide to spring tide. The direction of Euler residual current is seaward，and the direction of Stokes residual current is landward in the study area. The size and direction of the single-width water delivery are mainly determined by the Euler residual current. 2）During the dry season，the advection transport term is the principal constituent of the suspended sediment transport in Sanmen Bay，and the largest contribution rate of T1 is 83% in the study area. The contribution rate of the tidal pumping transport term （T3+T4+T5）is the largest in the middle of the bay. During the flood season，the contribution rate of the advection transport item in Sanmen Bay decreases，while the contribution rate of the tidal pumping transport term increases. 3）Regardless of the flood season and the dry season，the net transport of sediment at individual stations （SM08 and SM11） in Sanmen Bay is in the opposite direction，and the laws of other stations are basically the same.