Siltation in the rivers downstream of floodgates is serious in China, making the discharge capacity of rivers decrease obviously and threatening the flood control in upper rivers. Tidal wave deformation is one conclusive dynamic factor leading to siltation downstream floodgates in estuaries. The hydrodynamic characteristics of tidal wave deformation are different with different river lengths downstream floodgates, so the fluvial processes and morphology characteristics are also different. A 2D numerical model is established to simulate the tidal wave deformation with different-length river downstream the floodgate. The changes of tidal level, flow velocity and phase difference are simulated before and after the construction of floodgate. The length of river downstream floodgates influences tidal wave deformation obviously, and several conclusions are obtained to provide a theoretical basis for the prediction and analysis of the siltation characteristics in the lower reach caused by floodgates with different lengths in estuaries.