According to Fresnel formula and power reflectance, the sensing principle of optical fiber refractive index sensor based on core diameter mismatch is analyzed. The sensor consists of a multimode fiber in which a short section of standard single-mode fiber is inserted. To validate the theoretical result, output light power of the sensor is studied by glycerin aquatic solution experiments with different refractive indexes. The results show that output light power of the sensor strongly depends on the refractive index of the external medium. The sensor exhibits output light power with no change in the refractive index range of 1.300～1.441. Output light power drops linearly in the range of 1.441～1.452, with slope of -155.91. As the refractive index of the external medium is close to that of single-mode fiber cladding, the corresponding output light power is almost zero. The glycerin aquatic solution experimental result agrees with the theoretical result, and the refractive index range of linear light power dropping is 1.442～1.454, with slope of -49.67. The method is simple and low-cost. All signals of the sensing system are transmitted by fiber. It can measure refractive index of toxic and hazardous solutions with high resolution, and is suitable for gas concentrations measurement in flammable and explosive, strong electromagnetic interference environments and other particular conditions.