Abstract: The steady-state dynamic responses of a spherical cavity in nearly saturated fractional derivative viscoelastic soil are investigated subject to the internal water pressure in the frequency domain. By a stress coefficient depending on the porosity of soil, the values of internal water pressure in medium and in pore water are determined, respectively. The soil skeleton and the lining are treated as a viscoelastic medium with fractional derivative constitutive relations and a porous flexible material, respectively. Based on Boit's theory, the analytical solutions for the displacement, stress and pore water pressure of the partial sealed spherical cavity in the nearly saturated soil subject to the internal water pressure are obtained by the displacement potential functions. The dynamic responses of the spherical cavity are analyzed for different physical and geometry parameters. It is shown that the dynamic behaviors of the soil are described reasonably by the fractional derivative constitutive model. The saturation degree has great influences on the stress and pore water pressure, while it has a smaller impact on the displacement.