A new refractive index sensing scheme based on the cladding modes of fiber Bragg grating (FBG) is presented. In the experiments, glycerin aquatic solutions with different concertrations are used as the external refractive index sensing media, and the cladding diameter of FBG is reduced by etching the fiber with HF solution to enhance the sensitivity of cladding modes to the external refractive index. Dependence of the coupling wavelength of the etched FBG cladding mode on the external refractive index is measured. The results show that in the refractive index range of 1.3300～1.4584, the coupling wavelength of cladding mode shifts longer with the increase of the external refractive index and a higher sensitivity is achieved when the external refractive index is close to that of fiber cladding's, and the maximum refractive index sensitivity reaches 172 nm/riu (refractive index unit). Furthermore, the full-width-at-half-maximum (FWHM) of resonant cladding mode is 0.07 nm, which is about 1/4 of that of the core mode's. With this narrower spectrum, a higher sensing resolution can be achieved.