A novel continuous-wave fiber optics cavity ring-down refractive index sensing method based on the frequency-shifted interferometry is researched. We made a sensor by using a section of fused fiber optics taper in the fiber ring-down cavity to experimentally study the refractive index of liquids at different concentrations. The experimental results show that the detection sensitivities of NaCl and glucose solutions are 3.779 dB·RIU -1 and 6.413 dB·RIU -1 (RIU is refractivity unit), respectively, in the range of solution mass fraction less than 9%, and the minimum detection limit of the system is 10 -4. The relative deviation of measured cavity loss is less than 0.4% over 40 min continuous observation, showing good stability. In contrast to conventional cavity ring-down schemes, the frequency-shifted interferometry does not require fast detection electronics and reduces the need for hardware devices. The sensing system has the advantages of simple structure, low cost and high sensitivity, and has some application prospects of chemical detection and biosensing.