Although the polyolefin film porosity is known to affect the material's mechanical properties, insulation, and penetration, few accurate methods are available for its nondestructive detection. In this study, the effective permittivity is used as a bridge for establishing four equivalent models between the equivalent refractive index and porosity of materials. By practically testing the terahertz time-domain spectrum of the battery separator and microporous filter membrane, the refractive index, permittivity, porosity, and other material properties are quantitatively obtained. The results show that for a biphasic medium comprising the polyolefin material and air, the porosity and permittivity are highly correlated. The average relative error is approximately 2.53% between the porosities measured using the improved effective medium model and conventional gas displacement method; therefore, terahertz-spectroscopy-based porosity measurements are feasible for polyolefin films and should become a supplementary film-detection method.