By studying the exit angle of light after passing through the prismatic absorbing medium, the relationship between among the exit angle, the incident angle, the real and imaginary parts of the complex refractive index in the glancing process can be obtained. Numerical simulation calculations show that when the apex angle and refractive index of the prism are constant and the imaginary part is larger, the greater the difference between the exit angle of the absorbing prism and the transparent prism, the greater the influence on the refractive index calculated by the inversion. When the apex angle is π/3 and the absorption imaginary part is 0.20, 0.10, and 0.01, the maximum relative errors of the calculated refractive index are about 5.0%, 2.5%, and 0.2%, respectively. When the imaginary part is constant, the smaller the output in the case of a large angle, the error caused by a prism with a large apex angle is small, and in the case of a large exit angle, the error caused by a prism with a small apex angle is small.