Abstract: In order to analyze the electromagnetic shielding performance of optical windows of different substrate materials, based on Kohin's equivalent thin film model, the electromagnetic wave interface reflection coefficient was obtained considering the multiple reflections and refractions of electromagnetic waves between two interfaces in material. The matlab was used to calculate the shielding efficiency curves of the same mesh, different thicknesses and different materials, and the influence of thickness and material on the shielding efficiency of the light windows was analyzed. To verify the correctness of the simulation data, the metallic mesh with line width of 15 um, period of 500 um and resistance of ≤20 Ω was fabricated on ZnS, and the shielding effectiveness at 8 GHz～18 GHz was tested. Comparison results show that the testing results are in good agreement with the theoretical calculation data, and the error of the curves is about 2 dB to 4 dB. The result can estimate the shielding effectiveness of optical windows with metallic mesh coating, which canprovide references for the shielding effectiveness of optical window design subsequently.