A surface plasmon polariton (SPP) phase imaging microscope with a sub-wavelength grating structure is developed for high-resolution in-plane image measurement, which can be used on biological samples. Conventionally, most of SPP image systems use prism couplers to induce surface plasmons (SPs), but this approach has some drawbacks such as non-normal incident light producing optical aberration in imaging and making the metrology instrument more complicate. It can be improved by utilizing a normal incident light to excite the SPs through subwavelength grating structure, which replaces the prism so that it can observe in-plane sample on the sensing surface and simplify the instrument. Instead of measuring the intensity of the reflectivity, the phase measurement with higher sensitivity is proposed. In this study, the proposed SPP microscope integrates a common-path phase-shift interferometry (PSI) technique to obtain the two-dimensional spatial phase variation caused by biomolecular interactions on the sensing surface without requiring additional labeling. The common-path PSI technique provides long-term stability, even when it is subjected to external disturbances, to match the requirements of biomolecular interaction analysis. The system is presented as a high stability, high sensitivity, and in-plane SPP phase image.