With the tides propagating from the open sea to the lagoon,the mean water level (MWL) in the inlet and lagoon becomes different from that at the open sea,and a setup/setdown is generated. The change of MWL (setup/setdown) in the system imposes a great impact on regulating the development of tidal marshes,on determining the long-term water level for harbor maintenance,on the planning for the water front development with the flood control for the possible inundation,and on the interpretation of the historical sea level change when using tidal marsh peat deposits in the lagoon as the indicator for open sea’s sea level. In this case study on the mechanisms which control the setup/setdown in Xincun Inlet,Hainan in China,the 2-D barotropic mode of Eulerian-Lagrangian CIRCulation (ELCIRC) model was utilized. After model calibration and verification,a series of numerical experiments were conducted to examine the effects of bottom friction and advection terms,wetting and drying of intertidal areas,bathymetry and boundary conditions on the setup/setdown in the system. The modeling results show that setup occurs over the inlet and lagoon areas with an order of one tenth of the tide range at the entrance. The larger the bottom friction is,a larger setup is generated. Without the advection term,the setup is reduced due to a decrease of water level gradient to compensate for the disappearance of the advection term. Even without overtides,a setup can still be developed in the system. Sea level rise and dredging in the inlet and tidal channel can cause a decrease of setup in the system,whereas shoaling of the system can increase the setup. The uniqueness of the Xincun Inlet with respect to MWL change is that there is no evident setdown in the inlet,which can be attributed to the complex geometry and bathymetry associated with the inlet system. With the tides propagating from the open sea to the lagoon, the mean water level (MWL) in the inlet and lagoon becomes different from that at the open sea, and a setup / setdown is generated. The change of MWL (setup / setdown) in the system imposes a great impact on regulating the development of tidal marshes, on determining the long-term water level for harbor maintenance, on the planning for the water front development with the flood control for the possible inundation, and on the interpretation of the historical sea level change when using tidal marsh peat deposits in the lagoon as the indicator for open sea’s sea level. In this case study on the mechanisms which control the setup / setdown in Xincun Inlet, Hainan in China, the 2-D barotropic mode of After model calibration and verification, a series of numerical experiments were conducted to examine the effects of bottom friction and advection terms, wetting and drying of intertidal ar eas, bathymetry and boundary conditions on the setup / setdown in the system. The modeling results show that setup occurs over the inlet and lagoon areas with an order of one tenth of the tide range at the entrance. The larger the bottom friction is, a Without the advection term, the setup is reduced due to a decrease of water level gradient to compensate for the disappearance of the advection term. Even without overtides, a setup can still be developed in the system. Sea level rise and dingging in the inlet and tidal channel can cause a decrease of setup in the system, but shoaling of the system can increase the setup. which uniqueness of the Xincun Inlet with respect to MWL change is that there is no evident setdown in the inlet, which can be attributed to the complex geometry and bathymetry associated with the inlet system.