Surface plasmon resonance (SPR) sensing technique has a very high refractive-index (RI) resolution. However, this RI resolution is very sensitive to the thickness of metal films, deteriorating the adaptability of SPR sensors. The sensitivity of RI resolution of a spectral SPR sensor to the thickness of metal films is reduced by a technique called "polarization interferometry (PI)". That the PI technique could reduce the minimum of the SPR spectrum is experimentally demonstrated, and the RI resolution of the sensors with non-optimal metal films is improved. RI resolution ranging from 3.9×10-7 to 8.1×10-7 refractive index unit (RIU) is achieved with the thickness of the Au film ranging from 28.16 to 54.38 nm.