Hydrophobic fumed silica is prepared by a chemical reaction between hydrophilic fumed silica and active silane (such as chlorosilane or hexamethyldisilane). It has hydrophobicity (hydrophobicity) and cannot be dispersed in water. In order to solve some special technical problems in industry, various types of hydrophobic gas-phase silica have been developed. If hydrophobic gas-phase silica is produced by treating modified hydrophilic grade gas-phase silica with silane or siloxane, in the final product, the chemical treatment agent is bonded to the original hydrophilic oxide through chemical bonds. In addition to the above advantages of hydrophilic products, hydrophobic fumed silica products are characterized by low moisture absorption, good dispersibility, and rheological adjustment ability even for polar systems. Some products undergo structural modification on the basis of hydrophobic treatment, which can provide further assistance for customers to develop new products and improve their performance. For example, in a liquid system, hydrophobic gas-phase silica can reach a high dosage, with little effect on the viscosity of the system.