A modified diffusion model for polymer-solvent systems was proposed by introducing the Simha-Somcynsky（SS）equation-of-state（EOS）into the Vrentas-Duda free-volume model.The complex process of viscoelastic measurements in the Vrentas-Duda model was avoided successfully，since the effective free volume of a polymer can be estimated from the SS EOS together with the Williams-Landel-Ferry（WLF）fractional free volume.Calculated diffusion coefficients of benzene，toluene，ethylbenzene and chloroform in polystyrene，polyisobutylene and poly（vinyl acetate）indicated that predictions from the modified model were consistent with experimental values，and the accuracy was better than those from the Vrentas-Duda model for predicting solvent self-diffusion coefficients.Moreover，the modified model can reflect the effect of pressure on diffusion coefficients.Solvent diffusion coefficients in polymers decrease with increasing pressure.Especially in dilute polymer-solvent systems，diffusion coefficients change by several orders of magnitude with increasing pressure.With the increase of solvent concentration，diffusion coefficients gradually approach those values under atmospheric pressure，thus the effect of pressure can be neglected reasonably.