Abstract: In this paper, the methods for effective refractive indices of multi-phase medium are applied to study the relationship between the effective refractive indices and pore-ratio of nanoporous polymer films made of PS,PMMA and nanopores. And the formula for the relationship between the effective refractive indices and pore-ratio of nanoporous polymer films has been obtained. What's more, the results of the formula are in coincidence with the experimental results.