To achieve highsensitivity refractive index measurement, an optical fiber refractive index sensor based on doublemode interference is designed. The sensor is a singlemodemultimodesinglemode fiber (SMS) structure, in which the singlemode fiber and the multimode fiber are of the same diameter and core refractive index. To strip the cladding of the multimode fiber and place it in the environment to be measured, interference will occur between the two modes excited out of the multimode fiber. The transmission dips of the sensor will shift when the ambient refractive index changes, so the simultaneous measurement of the refractive index can be achieved by monitoring the wavelength shifts. Using FDTD Solutions software for simulation, the interference spectra at different refractive index were obtained. The results show that the sensitivity of the dip at 1517.32nm to the refractive index is 1848.2nm/RIU, and the minimum resolution error is only 2.74×10-5RIU. Compared with the traditional optical fiber sensor, the sensor processes simple structure, high sensitivity and low resolution, making it has high potential applications.