The effect of an electric field on the optical characteristics of bulk n-InSb in the terahertz spectral range (wavelength of 119 μm) at a temperature of 78 K was experimentally studied. The observed changes in the absorption coefficient and refractive index are explained by an increase in the average energy of electrons in an electric field, which is accompanied by a change in the average inverse effective mass of electrons due to the non-parabolicity of the conduction band. Along with the isotropic change in the optical characteristics in electric field, a dependence of the absorption and refraction of terahertz radiation on the direction of the polarization vector was revealed: the measurement results are different for electromagnetic radiation linearly polarized parallel and perpendicular to the field. A comparison of the experimental results with the results of theoretical modeling showed that the observed anisotropy of absorption and refraction is associated with the combined action of two factors: the non-parabolicity of the conduction band and the anisotropy of the nonequilibrium distribution function of electrons over states in momentum space.