Stray radiation analysis coupled with a temperature field is performed for a semitransparent window, focusing on the temperature-dependent optical properties. The transient temperature response of the optical window-based encapsulation structure is first investigated under an external transient high-heat flux loading. The spectral selectivity of the window to thermal radiation is involved. Subsequently, several typical cases for stray radiation are conducted, considering the inhomogeneous optical properties caused by the temperature heterogeneity. It is found that the stray radiation distribution is more chaotic compared to the results with optical properties independent of temperature. In addition, the stray radiation power has a large deviation (150%) if one neglects the temperature dependence. Meanwhile, the difference in wave band power decreases with the wavelength rising. Additionally, the stray radiation power generated by the window is far less in the visible wave band than that in the near-infrared wave band. The results reveal that the temperature dependence in optical properties of a semitransparent window should be seriously considered when calculating the stray radiation, especially in high-precision detection devices.