This paper proposes a new method for measuring film thickness that combines frequency-domain white-light interference and reflection spectroscopy. In addition to the film to be measured, this method considers air as a medium layer to construct a multilayer-film-structure model. The nonlinear phase is reconstructed using white-light frequency-domain interference fringes generated by the reflection of the multilayer film structure, and the film thickness is obtained via fitting. To solve the local optimal solution problem that occurs during fitting, the thickness obtained via reflection spectroscopy is used to limit the fitting range. In this study, theoretical derivation and numerical simulation are performed to examine the process of measuring the thickness of the film to be measured using a multilayer-film-structure model. Subsequently, a fiber-optic measuring system is constructed to verify the feasibility of this method by experimentally measuring the thickness of a single-layer silica film.