Potassium Dihydrogen Phosphate (KDP) and Potassium Dideuterium Phosphate (DKDP) crystals, which were used as fiequency converters for high power lasers, are crucial optical components for providing a short wavelength laser.However, exposure of KDP/DKDP crystals to the atmosphere environment for prolonged periods of time results in deliquescence and fogging of the surfaces, with a subsequent loss in conversion efficiency and a substantial reduction of the optical quality of the crystals.Therefore, a protective coating with high laser-damage resistance and good optical quality would be desperately needed.In this study, a protective coating was easily prepared from polysiloxane glass resin.The optical transparency, moisture-proof property and laser induced damage threshold (LIDT) were studied.The resulting coating showed a refractive index of ~1.42 and LIDT of 28 J/cm2 (1064 nm, 3us).A porous sol-gel silica coating was also prepared for experimental comparative study.Moisture-proof tests showed that the protective coating could keep quite stable optical properties for 3 months in an ambient with relative humidity higher than 90%.