The finite difference time domain(FDTD) method is used to construct the model of nanoporous film and to simulate the propagation of plane lightwave in the film and calculate some refractive indices according to different ratios of pore. From these simulation results, it is found that equivalent refractive index of nanoporous film will decrease with increase of porous ratio. By using equivalent dielectric constant theory of composite media in solid-state matercal science a theoretical model called composite media in parallel is proposed to explain the connection between the porous ratio and equivalent refractive index of the nanoporous film, and function expression between them is obtained. With the comparison between the function curve and the simulation results, it is found that the function curve based on the theoretical model of composite media in parallel is in accord with the results simulated by FDTD method.