A novel optical fiber refractive sensor is proposed and demonstrated experimentally. It is based on side polished fiber Bragg grating (SPFBG) manufactured by wheel side polishing method. When about half of SPFBG is overlaid by refractive index liquids, the reflective Bragg wavelength is split into two. The difference between the two reflective Bragg wavelengths has related with the refractive index of overlaid material, but their strain responses are the same, which have no-reference to the ambient temperature. Hence the difference between two reflective wavelengths can be used to measure the changes of the outer refractive index accurately, and to reduce the effect of strain during the measurement. The experiments show that the resolution of the sensor is 0.0001 with outer refractive index in the range of 1.4298 to 1.4479.