Single Ta2O5 and double layer Ta2O5/SiO2 films are deposited on Si and fused silica substrates by dual-ion-beam sputtering. With the Cauchy dispersion model, the optical constants of Ta2O5 and SiO2 thin films are obtained by fitting the transmission spectra of single Ta2O5 and double layer Ta2O5/SiO2 films using the combination of simplex and genetic algorithm (GA) optimization method. It shows that the results calculated by fitting the whole optical spectra are well consistent with the measuring values. The refractive index error and thickness error of single layer Ta2O5 film are less than 0.001 nm and 1 nm, respectively. In the case of double layer Ta2O5/SiO2 films，the maximum refractive index error and thickness error are in the range of 0.004 nm and 2.5 nm, respectively. In addition, the deposited double layer Ta2O5/SiO2 thin films are treated at 400 ℃ in air. The changes of microstructure, surface morphology and optical properties of the Ta2O5/SiO2 films are investigated.