In this study, the mode equation of an optical fiber-assisted circular thin-film waveguide was deduced theoretically based on a planar waveguide approximate model, from which the discrete thin-film waveguide modes are obtained. Combined with the full vector mode theory, new insights into optical fiber mode transition are obtained. It is found that the fiber cladding mode is transited into the phase-matched thin-film cladding mode supported by the circular thin-film waveguide with a certain thickness, in the mode order from high- to low-order rather than the well-known low- to high-order. The further increase of thickness of the circular thin-film waveguide eliminates the phase-matching condition and the generated thin-film cladding mode transforms back to the adjacent low-order optical fiber cladding mode, resulting in a periodic mode conversion process. These theoretical results were validated by analyzing the spectrum evolution of an indium tin oxide film-coated tilted fiber Bragg grating. An asynchronous excitation process with the same period of the orthogonally polarized thin-film waveguide modes (P- and S-polarizationand) and its periodic tuning property for optical fiber polarization are realized. These results are consistent with those of previous experiments and are expected to provide a new method for fiber-optic vector parameter sensing (e.g., vibration, twisting, pressure, acoustic field, etc.), fiber-optic biosensing (discriminative sensing of volume refractive index and surface refractive index), and polarization-dependent optical communication filters.