The refractive index sensing characteristics of the elliptical core polarization maitaining fiber(Ecore PMF) based on intermodal interference are investigated. The refractive index sensor probe is composed with a section of E- core PMF that etached by hydrofluoric acid. And the taper structure of sensor probe can be controlled by erosion time conveniently. The relationship of the two-lobe intensity distribution produced by the intermodal interference between the basic made and the first order higher mode of PMF with the analyte refractive index is analyzed comprehensively. Furthermore, the influence of the tapered area radius and the wavelength on the refractive index sensitivity is discussed deeply. The experiments demonstrate that the refractive index sensitivity of a refractive index sensor with 8 μm tapered area radius can reach - 7.1/RIU, which means the ratio between the normalized ralue and the refractive index unit (RIU), near 1.39 refractive index, when the laser source wavelength is 980 nm.