Fiber optic refractive index sensor is researched and a side-polished multimode fiber (MMF) based refractometer is proposed in this work.In the refractive index range from 1.300to 1.430,the transmission spectrum and optical power property are studied with the partial si de polished multimode fiber core diameter D of 50μm,62.5μm and 105.0μm and differen t residual radii R (the minimum distance between the side-polished surface and the center of multimode fiber).The results show that the transmission spectra have no corresponding drift when the side-polished multimode fiber is covered with diff erent refractive index materials, but the output optical power has larger changes.Therefore,the optical power is used to study the index sensing properties of the partial side polished multimode fiber.The experimental results indicate that the side-polished multimode fiber based refractometer is sensitive to the refractive index of environmental material in the refractive index range of 1.300-1.430.A refractometer with lower D and residua l radius R has a higher sensitivity,the highest sensitiv ity can be up to 42.23dB/RIU and the minimum r esolution is 2.37×10－5RIU while R is 0μm and D is 50μm.And the sensitivity of the refractometer can still be 10－5RIU when D is 105.0μm and R is 30μm.The side-polished multimode fiber base d refractometer is useful for biological sensors.