Thermal-optical properties of optical window in complex environment are analyzed. From the perspective of considering thermal deformation of optical window, fused silica is selected for material of the window. In order to calculate temperature distribution of optical window within a work cycle, heat flux is assigned on a scale of weight to outer surface of the window and radiation of the window is taken into account. Carrier of the window is an unmanned aerial vehicle with high-altitude and high-speed. Temperature of the window′s external surface increases sharply due to the aerodynamic heating that generated during the high-speed flight. Since thermal conductivity of fused silica is very small, a great axial temperature difference produces at the window. Heat deformation of the optical window is calculated when axial temperature difference is 55 ℃, 70 ℃ and 90 ℃, respectively. Radiuses of curvature of internal and external surface of optical window are obtained approximately by rules of its deformation. Optical path difference caused by the deformation of the surface shape and refractive index changes is analyzed. Zernike polynomial coefficients are fitted. Finally, the Zernike polynomials are put into the Code V to assess the optical properties of window glass. With the introduction of wave-front aberration, change of defocus is -0.114 mm and maximum decline value of modulation transfer function (MTF) is less than 0.01. The results show that the optical window meets optical performance requirements.