The patent abstract shows that the present invention relates to the field of nanomaterial preparation technology, and specifically to a method for preparing reactive hydrophobic silica. It comprises the following steps: (1) adding water to dimethyldichlorosilane to carry out a hydrolysis reaction to obtain an incomplete hydrolyzate; (2) introducing ammonia into the incomplete hydrolyzate to react, filter, and obtain a hydrophobic treatment agent; (3) uniformly mixing the hydrophilic silica and the hydrophobic treatment agent, heating and stirring the reaction, and vacuum treating to obtain a reactive hydrophobic silica. The raw materials used in the synthesis of the treatment agent of the present invention are common raw materials, which have been industrialized. The raw materials are easy to obtain and low in cost, and the surface of the silica is effectively reactively grafted. The hydrophobic silica obtained by the present invention can be prepared at low temperature, has a high surface grafting rate, and can be used as a structuring control agent for silicone rubber.