On the basis of measurement and theoretical analysis，optical and nonlinear optical properties of the Au：TiO2 and Au：Al2O3 composite films with different Au nanoparticle size and different substrates were investigated. The relationship between the Au nanoparticle size and substrates and the surface plasmon resonance (SPR) of Au composite films were characterized by UV-VIS-NIR spectroscopy，and the dependence of third-order nonlinear susceptibility of the composite films on Au nanoparticle size and substrates was investigated by picosecond Z-scan technique with different exciting wavelengths (532nm and 1064nm). The analysis of optical properties of these films was carried out using the theories of surface plasmon resonance and localized field enhancement，respectively. The experimental and theoretical results revealed the dependence of third-order nonlinear susceptibilities χ(3) on Au nanoparticle size and substrates. For Au nanoparticle of size around 35nm embedded in TiO2 substrate with larger permittivity and the exciting wavelength at 532nm close to the surface plasmon resonance (618nm—632nm)，the third-order nonlinear susceptibility χ(3) showed a maximum value of 25×10-9esu.