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In the letter, we propose a novel refractometer based on a microstructure fiber (MF) to measure water-samples with a large refractive index. The designed MF is a hollow-core fiber and supports only fundamental mode when the MF is filled with a tested liquid whose refractive index is larger than 1.45. The fundamental mode's effective index of the filled MF is linearly proportional to the refractive index of the liquid. Utilizing this property of the designed MF, the Mach-Zehnder interferometer based sensor is realized. The theoretical result shows that the proposed sensor can works well in the refractive index range of 1.48 to 1.60 and the sensitivity of the proposed refractometer is about 6.41*10-6.