Although the porous silica antireflective (AR) coating has high laser damage threshold after heat treatment, its transmission is correlated with environment relative humidity due to many Si-OH hydrophilic groups on the coating surface. The aim of the experiment is to modify internal component, enhance the hydrophobic performance and improve the transmission stability of the films. In this paper, Si-O-Si(CH3)3 non-polar hydrophobic groups appear on the coating surface through atmosphere treatment of ammonia and HMDS (hexamethyl-disilazane), causing hydrophobic performance, thus the stability and life of AR coating are improved greatly. This is very important for coating to use in laser device. The results also show that the coating surface is smooth after above treatment, and its roughness is estimated to be 3.575 for root-mean-square value and 2.850 for average value which meets the precision requirement of large scale laser.