Optical filters are crucial devices for wavelength channel selecting in WDM fiber-optic communication networks. To measure the inner physical parameters of optical filters is significant for analyzing emerging new and more complex structures. A general and easy-to-operate method is proposed to extract physical parameters for complex filter structures with optical low-coherence measurement. By treating optical filters with digital filter concepts and Z-transform analysis, we can build the relation between interferogram peak values and transfer function coefficients with brief linear equations. These equations can be used to derive the exact expression of transfer functions, which can manipulate the physical parameters. For a given filter structure, according to relations between transfer functions and physical parameters, the physical parameters could be extracted. This is especially meaningful in the post–fabrication process.