Refractometry based on multimode optical fiber with large core is presented. The multimode fiber (MMF) with the cladding/core radius 62.5/50 μm is connected with normal single mode fiber,which composes a single-multi-single (SMS) mode fiber structure. The structure is sensitive to the surrounding refractive index and is used as a refractometry. The demoducation system is made of the broadband amplified spontaneous emission (ASE) source and optical spectral analysis (OSA). A simple theoretical analysis of the refractometry is given and a HF corrosion method is proposed to enhance the performance of the refractometry. Experimental results prove the feasibility of the refractometry.