SiO2/AC composites were prepared in sol-gel method with tetraethoxysilane (TEOS) as the precursor and active carbon (AC) as the carbon resource. XRD, SEM, BET and electromagnetism parameter analyses showed that the structures, electrical conductivity and electromagnetism parameter varied with the content of SiO2 in SiO2/AC composites, and the microwave absorption could be improved with SiO2 content. It has been found that the active carbon and TEOS will form a homogeneous mixture formation through sol-gel method, the reaction velocity is slower and the generated SiO2 particles are smaller when the temperature is low. The electromagnetic parameters and electromagnetic wave absorption properties of SiO2/AC composites will change if the content of SiO2 is changed. When the content of SiO2 is 12%, the absorption property is the best.