A new method based on the polarization interferometer structure has been applied to measure the optical admittance, the refractive index and thickness of a thin film. The structure is a vibration insensitive optical system. There is one Twyman-Green interferometer part to induce a phase difference and one Fizeau interferometer part to induce the interference in the system. The intensities coming from four different polarizers were measured at the same time to prevent mechanical vibration influence. Using the polarization interferometer, the optical admittance, the refractive index and thickness of a single layer of Ta2O5 thin film has been measured. The measurement results were compared with the results obtained by ellipsometer. The results meet reasonable values in both refractive index and thickness.