A refractive index sensor based on single-mode no-core single-mode fiber structure is proposed and experimentally demonstrated.When the difference between surroundin g refractive index (SRI) and the refractive index of no-core fiber (NCF) is relatively small,the NCF can be treated as a weakly guiding multimode fiber with step-index profile,consttructing the traditional single-mode multimode single-mode (SMS) fiber structure.Under the weakly guiding condition,the mode excitation and the output characteristics of this sensor are analyzed by using linear polarization mode approximation method.And numerical calculations are carried out based on the theoretical anal ysis.Numerical simulations show that the characteristic wavelength in the output spectrum will shift as the SRI changes, which reflects the refractive index sensing capability of this fiber structure. For verifying testify the conclusion of the numerical simulation,several experiments have been carried ou t.As the SRI increases, the characteristic wavelength shows blue shift correspondingly,which agrees well with the numerical simulation and the theoretical anticipation.The sensitivity of this sensor obta ined from experiments reaches 205.42nm/RIU for a refractive index range from 1.3to 1.4.And a furthe r confirmatory experiment for testing the absolute ethylalcohol in room temperature indicates tha t such a refractive index sensor′s relative error is 0.1%.