Bulk inhomogeneity of refractive index is a normal kind of film defects, the reverse determination of which can be vital for the design and manufacture of coatings. The characteristic matrix of inhomogeneous thin films is derived, and an approximation model for the spectral characteristics calculation of the thin films at oblique incidence is presented using the matrix method. The calculation accuracy and time consumption of the presented approximation model are discussed in details. The effectiveness of the model is proved by calculating the ellipsometric angle. It turns out that the model for inhomogeneous films at oblique incidence presents a rapid and effective tool for the numerical optimization algorithm application into the data fitting of measured broadband spectral characteristics of actual multilayer coatings.