Optically transparent frequency selective surface (FSS) is designed on metallic mesh. When it is loaded on the optical window of combined guidance radome, the diffractive effect to infrared will interfere with the quality of imaging system. For reducing the interference in the design procedure of optically transparent FSS, this paper specifically studies the influence of structural parameters of optically transparent FSS on the diffractive characteristics based on Fraunhofer diffractive theory. It is found that when the period of metallic mesh increases or the line width decreases, the intensity of diffractive light increases but distribution remains constant and the period has more notable influence on intensity. As the slit width and length of cross FSS unit increase separately, the diffractive light has rising intensity and unchanged distribution. Moreover, when the wavelength of incident light increases, the integral transmittance of optically transparent FSS increases. These show a good agreement with experimental results. The analysis mentioned above will have an important guiding significance in the process of designing optically transparent FSS.