The mathematical model of the 2D-system of electrons in the subsurface space of the homogeneous narrow-gap semiconductor was developed for accumulation layers. The calculation of the 2D-systems parameters was carried out by numerical self-consistent integration of the Schr"odinger and Poisson equations by using the Fermi and quasi-classical (WKB) descriptions of the eigenfunctions of the continuous spectrum - the states of electrons "in continuum". par It is shown that the quasi-classical approximation is preferable in comparison with the Fermi one for the description of the continuum for 2D-systems. The parameters of the two-dimensional gas were computed in wide range of temperatures (200---300,K) and potentials (0---0.2,V). There exists possibility of experimental observation of quantum subbands in accumulation layers in the subsurface space of the narrow-gap semiconductor at room temperature.