Au/SiO2 nano-composite multilayer thin films were prepared by magnetron plasma sputtering. The effect of microstructure on the absorption spectra of films of the same thickness but under different deposition conditions, and films deposited under the same conditions but having different thicknesses was investigated. The results show that the optical absorption peaks appeared at 560 nm for the Au/SiO2 nano-composite multilayer films, the intensity of the absorption peak increased with increasing Au content and thickness of films. No nano-multilayer structure and absorption peak was observed when Au particle connected together due to high Au content. The optical absorption spectra of Au/SiO2 multilayer thin films well agree with the theoretical optical absorption spectra calculated from modified Maxwell-Garnett effective medium theory.