Based on the thermal optical analysis,the glass thickness of optical window under the complex environment is optimized. Intensity and the thermal environment of optical window is analyzed. steady-state temperature field is calculated. By mapping the temperature field to structural model,the deformation of optical window under the force-thermal coupling condition is calculated. Combinating refractive index gradient analysis and glass surface change of optical window,thermal optical analysis is carried out. When the glass thickness is different,the optical path in root mean square error(RMS) of optical window with diameter of 350 mm is calculated. Thickness of the optical window is determined as 18 mm. The results show that the optical window can not only meet the strength and reliability requirements,but also meet the optic index in the effective optical aperture. These results provide the guidance for the optical window design.