We derive the phase difference equation between the - and -polarizations of reflection light based on the Kretschmann-Raether configuration. This phase difference equation is used to examine the relationship of the incident angle and metal film thickness versus the phase differences under a small refractive index variation. For a fixed incident angle, the phase difference has a higher measurement sensitivity than the reflectivity change. At the critical angle, there is a higher sensitivity when the metal film thickness is smaller than the skin depth. The surface plasmon resonant (SPR) angle dominates when the metal film thickness is greater than the skin depth. The phase measuring sensitivity at the SPR angle is higher than that at the critical angle by 1 order.