In order to further understand the relations between refractive index and porosity in mixed films，We did the experiments of mixed film system TiO₂:Ta₂O₅, using coevaporation. On the basis of our experimental results，we put forward an empirical porosity expression to fit the data of TiO₂:Ta₂O₅ system，and we also improved the calculating procedure of the refractive index of mixed films. The results of theoretical calculations using our empirical porosity expression agree with those of experiments. The results of experiments and theoretical analysis show that it is possible that the mixed films have less porosity than pure thin films，and those films cause a peak in the curve of refractive index of mixed films versus the ratio of the mixed weight.