Preparation of ultrafine silica by olivine acid dissolution is simple and low-cost,however,the ultrafine silica are liable to agglomerate with poor dispersing property due to strong hydrophilicity.Silane coupling agent was used to modify the ultrafine silica prepared by olivine acid dissolution.The effects of varieties and dosage of modifier,modifying time and temperature on the modification of silica were investigated by single factor method.The results show that the number of hydroxyl on the surface of ultrafine silica can be reduced to 2.08×10 ²⁰ units/g,the grafting rate is 8.35% and the contact angle is 101.51° when the addition of silane coupling agent KH-550 is 6 mL/g,modifying time is 5 h and reaction temperature is 75 ℃,which improves the hydrophobicity of ultrafine silica greatly.