An optical non-contact measurement with Sagnac interferometer has been considered as the best option compared to Michelson and Mach-Zehnder ones for less sensitive to the external environment. Signal analysis produces errors due to frequency-time domain using single detection and a self-referenced polarization and phase-shifting technique for real-time thickness measurement is proposed. A beam splitter placed at the interferometer output separated the beam into two identical linearly polarized beams. One of the beams served as a reference, while the other served as a sensing arm. The output linear polarizer set at 45◦ relative to a reference plane was positioned anterior to the photodetectors to get rotating light intensities for phase shift measurement; hence, the intensities at various polarizations of 0◦, 45◦, 90◦ and 135◦ were automatically acquired without any polarizing device adjustments. These intensities were then transformed into a phase retardation introduced by a sample, and the resulting phase shift was eventually converted into film thickness. The samples were properly prepared, with pure BK7 substrate being deposited by Tantalum pentoxide (Ta2O5). The real-time phase shift is successfully realized at 0.2 rad. This shows the impact of state of polarization thickness thin film of 259.32nm. The mathematical theory was effectively demonstrated and empirically confirmed.