Abstract: In the subsea tunnel construction, the seepage volume and the minimum rock cover are most important in the drainage design of the tunnel, and in the tunnel route selection. With the hydro-geological model of half infinite space, the seepage water volume is computed both by analytical method and numerical simulation, and the relation between seepage volume and rock cover thickness is analyzed. According to the rock cover thickness determined by the minimum seepage volume, a linear relation between sea depth and rock cover thickness is obtained.