By using a modified self-consistent variation method, we have calculated the subband structures of n-type inversion layer in p-type HgCdTe NIS (metal-insulator-semiconductor) devices, obtained the ground subband energy E0 and its dependence on both the doping concentration in bulk HgCdTe and the surface electron concent-ration in the inversion layer. In the treament, the influence of non-parabolic band structure due to the interaction between the conduction band and the valence band, the Zener tunneling, the resonant states and the screening effect of the surface electric field have been considered. A simple and useful formula for calculating gro-and state energy E0 has been presented. The calculated results show good agreement with experimental data.