One of the main reasons for the longitudinal deformation of rivercrossing tunnels is riverbed scouring and siltation. Taking the construction of Qiantang River Crossing Tunnel on No.2 line of Hangzhou Metro as an example, the authors present the mechanisms of the longitudinal deformation of rivercrossing tunnels caused by riverbed scouring and siltation by means of numerical simulation and theoretical calculation. Furthermore, they educe the longitudinal deformation curve of the tunnel by means of crosssection calculation, demonstrate the relation between the connection stiffness of the lining segments and the longitudinal deformation of the tunnel, and finally make some advice to control the  longitudinal deformation of the tunnel.