Abstract: The sheared ion flow instability is studied in an inhomogeneous plasma background of magnetopause boundary layer at the high latitude magnetotail. By considering tail-aligned currents, we find that the instability excitation strongly depend on the disturbed wavelength. The quasi-critical wave number for the instability is obtained. For relative long wavelength perturbation the instability tend to be excited at the inner edge of the boundary layer. The stable surface wave at magnetopause and the K-H instability at the inner edge of the boundary layer can exist at the same time, and that may help to transfer the momentum toward magnetosphere continuously.