A novel method to measure the absolute phase shift on reflection of thin film is presented utilizing a white-light interferometer in spectral domain. By applying Fourier transformation to the recorded spectral interference signal, we retrieve the spectral phase function \phi which is induced by three parts: the path length difference in air L the effective thickness of slightly dispersive cube beam splitter T_{eff} and the nonlinear phase function due to multi-reflection of the thin film structure. We utilize the fact that the overall optical path difference (OPD) is linearly dependent on the refractive index of the beam splitter to determine both L and T_{eff}. The spectral phase shift on reflection of thin film structure can be obtained by subtracting these two parts from \phi. We show theoretically and experimentally that our new method can provide a simple and fast solution in calculating the absolute spectral phase function of optical thin films, while still maintaining high accuracy.