The Au/SiO2 nanocomposite was produced using surface active agent P123, and its optical absorption spectra was measured by spectrophotometer. It was found that the intensity of absorption peak was strengthened and underwent a red-shift as the neutralization reaction time extended. The variation of optical extinction property of one gold particle (GP) with different size had been investigated by Mie theory. It was found that the extinction property of the single spherical GP possessed an obvious size effect. The extinction property of Au/SiO2 nanocomposite had been analyzed by Maxwell-Garnett (MG) theory. It showed that the extinction peak underwent a red-shift and the extinction intensity was strengthened when the size of gold nanoparticles (GNs) increased. The theoretical and the experimental results show that the extinction property of Au/SiO2 nanocomposite changes because the scattering effect of GNs are strengthened when their sizes increased by controlling the neutralization reaction time.