Five different concentrations of sodium silicate solutions were prepared from fly ash. The sodium silicate solutions were treated by sol-gel method, and hydrophobic SiO2 aerogel was obtained via ambient pressure drying. The influence of sodium silicate modulus on the structure of aerogels was investigated through measurements of density, specific surface area and contact angle. The chemical reaction mechanism of nucleophilic substitution SN1 for hydrophobic modification of aerogel surface was proposed. The results show that the hydrophobic performance of aerogel is directly related to the number of -Si-(CH3)3 attached to the surface. When sodium silicate modulus is equal to 2.50, the best hydrophobic performance of aerogel is achieved. The density and specific surface area of aerogel increase with the increase of sodium silicate modulus. When the sodium silicate modulus is equal to 0.75, the lowest density and specific surface area of aerogel are recorded, which are 0.073 9 g/cm3 and 588.5 m2/g, respectively.