A fiber refractive index sensor based on the combination of waist-enlarged tapers and singlemode-multimode-singlemode (SMS) fiber structure is demonstrated. A 18 mm multi-mode fiber (MMF) is spliced with waist-enlarged taper between two conventional single-de fibers (SMFs) to construct an all-fiber sensor. The effective refractive index of the cladding modes changes with the external refractive index. It shows that the sensitivity of the refractive index sensor is 152.237 nm/RIU (RIU is refractive index unit) in the refractive range index from 1.3725 to 1.4620. The temperature cross sensitivity of the refractive index sensor is 2.6×10-4 RIU/℃, so when the temperature changes little, the temperature impact on the refractive index measurement can be ignored. The presented refractive sensor is with simple manufacture and relatively low cost, so it has great potential for biological and chemical applications.