When hypersonic vehicles have flights in atmosphere, the temperature of the window of infrared (IR) detection systems rise rapidly, due to the aerodynamic heating by hypersonic airflow around. High-temperature IR windows dominate the aero-thermo-radiation effect, which reduce the performance of IR detection systems, including detection range, identification probability, tracking precision, and so on, or even make these systems fail. By simplifying radiation transfer model for IR windows, an experiment platform was designed to measure thermal-radiation characteristics of IR windows. On the platform, the experiment on thermal-radiation characteristics of a sapphire window were studied, in 3.7-4.8 μm. The research result indicates that, in 100-350 ℃, the higher the temperature of the sapphire IR window is, the smaller the transmittance is, and the stronger the self-radiation is. Both the transmittance and the self-radiation are related to the third power of the window temperature. From 100 ℃ to 350 ℃, the transmittance of the window drops about 16%, otherwise the self-radiation enhances above 100 times, of which the influence on the MWIR detection system is bigger than that of transmittance.