The hypersonic vehicle is subjected to intense aerodynamic heating during flight. The temperature of the sapphire infrared radiation (IR) window located in the head rises significantly, and the transmittance decreases while the self-radiation is greatly enhanced, resulting in a decrease in the internal infrared detection gas signal-to-noise ratio. The aero-heating with unsteady temperature rise process of the sapphire IR window and the infrared radiation transmission characteristics of the 3.7-4.8 μm band were numerically simulated. The results show that the average temperature can't accurately reflect the infrared transmission characteristics of the sapphire IR window and its impact on the sensitivity of the infrared detector. There is an optimal infrared detection window thickness in which the sensitivity of the infrared detector reaches best.