For eliminating the limits in the classical models, here we establish a general model for precisely predicting dielectric constant of porous materials. In this model,dielectric constant is independent on pore shapes when the pore size is far smaller than electromagnetic wavelengths; however, it depends on the porosities and correlation between the open pore direction and wave propagation direction. The structure factor β in the effective dielectric formula is 1 and 3 for through-hole and closed-pore materials, respectively. The experimental results are in good agreement with the model, suggesting a favorable tool for predicting dielectric properties of porous materials.