A fiber-optic refractometer based on the multimode interference in the singlemode-multimode core-singlemode fiber structure is demonstrated. The refractive index dependence of re-imaging resonant wavelength shift of the devices is investigated. It shows that the proposed sensor has a measurement resolution of 5.4×10-5 for the change of refractive indices from 1.336 to 1.372. The proposed sensor has great potential for biological and chemical applications.