Surface fogging induced degradation has been a bottleneck problem in potassium dihydrogen phosphate (KDP) crystals due to they are grown from aqueous solution. In this paper, we developed a facile method to prepare a double-layer antireflective coating with moisture-proof and laser damage resistant properties for KDP crystals. The bottom layer was a poly siloxane coating with dense structure and silanol side groups, while the top layer was a hexamethyldisilazane (HMDS) modified nanoporous SiO2 coating. Both of the cols were nonalkaline and nonaqueous to make sure those are harmless to KDP crystals. The double-layer coated KDP crystal exhibited a maximum transmittance of 99.9% with an average increase of transmitted light of 6-7% over the wavelength range between 351 and 1053 nm. After exposure in a 55% relative humidity environment for 6 months, the double-layer HMDS_SiO2/PS coating coated KDP crystal displayed nearly the same optical transmittance as the original one, whereas the single-layer HMDS_SiO2 coated KDP crystal had a transmittance loss of similar to 5%. Moreover, the laser-induced damage threshold of the double-layer coating on KDP crystal reached 11.5 J/cm(2) (355 nm, 3 ns). This multifunctional antireflective coating not only can be used for KDP crystals, but also can be applied to thermal-sensitive polymeric substrates.