The refractive index sensor with single mode-multimode-single (SMS) mode structure based on multimode interference effect usually needs cladding corrosion to improve the sensitivity, and it is vulnerable to the influence of temperature. To solve these problems, a novel fiber refractometer based on fiber core diameter mismatch multimode interference is proposed. The refractometer structured of single-mode fiber/dispersion compensation fiber/single-mode fiber (SMF-DCF-SMF) and a fiber Bragg grating (FBG) has a total length less than 100 mm. The experiment results show that the sensitivity of the sensor in measuring liquid with a refractive index of 1.33~1.39 is 232.8 nm, and the FBG has a good function of temperature calibration.