Aiming at the strong permeability of sandy embankment around the artificial island and the obvious influence of sea tide level on the water level of the inner river，a reasonable water level of the inner river on the artificial island should be selected to avoid artificial drainage. Through the establishment of unsteady seepage model，the relationship between seepage field and seepage flow changes with tidal level is studied. The typical tide level of the offshore sea is selected，and the net seepage discharge of various proposed inland river levels within a tidal cycle is calculated by subsection summation method. The water level corresponding to zero net seepage discharge is the optimal normal water level of the river channel，which provides a basis for determination of the scale of the inland river channel and the elevation of dredging ground in the island.