To meet the high demand for high precision refractive index measurement in different fields such as medicine and chemistry, based on high precision sweep spectrum measurement technology, combined with tilt fiber grating for refractive index measurement, a high precision refractive index change sensor system is proposed. The spectral measurement of the cut-off mode of tilted fiber grating is carried out under the condition of slightly different refractive index, and the sensitivity of the cut-off mode to the external refractive index response is 28.3 nm/RIU. The accuracy of measuring the peak center wavelength of a cut-off mode at a fixed refractive index is calibrated. The measurement accuracy of refractive index change of 1.15 × 10-6 RIU is obtained, and the measurement accuracy of 6.5 × 10-7 RIU can be further achieved in the case of static measurement.