We propose a surface plasmon resonance (SPR) sensor based on phase modulation and polarization interferometry, both of which provide a refractive index (RI) resolution of the same order as that of SPR sensors of the phase type. And it has a wide dynamic range and insensitivity of RI resolution to the thickness of metal films as that of the intensity type SPR sensors. In this letter, we choose electro-optic (EO) phase modulation instead of the angle modulation. We demonstrate theoretically that with the EO phase modulation, our sensor could provide a better RI resolution.