In this work, an improved model for simulation of molecular diffusion in porous materials with bidisperse pore structures was proposed under the linear adsorptive isotherm and solved by numerical methods based on the method of fourth order Runge-Kutta combined with the linear driving force (LDF) approximation. A criterion to identify the diffusion controlling step was proposed. The diffusional experiments were conducted on some prepared porous materials with bidisperse pore structures. Experimental data were predicted very well with the improved bidisperse pore model. A correlation equation of effective diffusivity as a function of the physical properties of porous materials with bidisperse pore structures was established. Experimental values of effective diffusivity available in the literatures can be interpreted better by the established correlation equation than those by the previous correlation equations.