The Han River estuary lies in the trans-boundary region of the North and South Korea. Thus as a result, it is often difficult to obtain continuous tidal data spanning over a long period of time. Thus, this paper proposed to set up a 2D Mike21FM numerical model to obtain accurate tidal level predictions of the Han River estuary in the Korean Demilitarized Zone. In order to obtain a good calibrated model, the location, shape and tidal stations’ influence of the open sea boundary is extensively examined. In addition, a varying roughness coefficient based on the bathymetry’s depth is used instead of a constant value for all depths. It is suggested that for future research, the model can be made more robust through further calibration using tidal forecasts as the input data for open sea boundary. This will eliminate the need for measured data for the open sea boundary in order to forcast accurate inland’s stations levels.