The Si-OH group on particle in the silica colloidal suspension is modified to Si-O-Si(CH3)3 non-polar hydrophobic group using hexa-methyl-disilazane (HMDS). The organic modified silica antireflective (AR) coatings that coated on potassium di-hydrogen phosphate (KDP) crystal and silica glass substrates by spinning method, has good moisture-resistant property. The transmission of coated substrate is above 99%. The optical roughness root-mean-square value is 0.94 nm and the water contact angle of coating is about 140°. The HMDS modifies SiO2 sol in non-polar solvents which need no treatment can be used in KDP crystals. In the condition of the room temperature and high humidity, the transmittance decrease of KDP crystals with the moisture-resistant(MR) AR coating is equal to the MR/AR two-layer coating. The reflection losses at three wavelengths from two surface of the tripling crystal can be minimized by spin coating.