A mathematical model for simulating the thermal radiation process of a laser-heated surface-absorbing approximate-transparent infrared window is demonstrated. The model is based on the heat conduction equation and Kirchhoff’s law. The simulation was conducted for a germanium window to calculate the radiometric quantities including radiance, exitance, and flux. The calculated results of the radiance reveal that the infrared radiation of the germanium window has approximately the Lambertian property. Numerical calculations demonstrate that following continuous irradiation of a 160-W/cm2 near-infrared laser for 500 ms, the maximum radiant exitance at the surface of the window reaches the order of magnitude of 1 W/m2 in the medium-wave infrared band and 10 W/m2 in the long-wave infrared band. A comparative study was conducted to investigate the radiant flux differences of intrinsic germanium, n-type doped germanium, and p-type doped germanium under the same conditions.