Ag-Au alloys/SiO2 composite thin films （nAg/nAu=1∶0，2∶1，1∶2，0∶1） were prepared with AgNO3，HAuCl4 and tetraethyl orthosilicate by means of sol-gel method and deoxidized using ultraviolet radiation. The results of scanning electron microscope and X-ray diffraction indicate that the composite thin films are very homogeneous and the size of the nanoparticles is about 10 nm. The results of the optical absorption spectra show that, with the increasing of nAg/nAu, the absorption peaks close to 430 nm, which belongs to the surface plasmon resonance absorption peak of Ag nanoparticles, are red-shifted to the viscinity of 605 and 880 nm, which belong to the surface plasmon resonance absorption peaks of Au nanoparticles. The surface plasma resonance absorption peaks around 515 and 730, 550 and 730 nm, belong to the samples （nAg∶nAu＝2∶1，1∶2），respectively，showing the formation of Ag-Au alloys dispersed in the SiO2.